(FORTUNE Magazine) – Easily the most wonderful moment in the history of the Americans With Disabilities Act (ADA) occurred on the Senate floor late in 1989 when Congress was debating the act, which will soon become operational in the business sector. (ADA takes effect for companies with 25 or more employees on July 26.) It seems that the original bill, like most federal civil rights legislation, exempted Congress itself from the requirements. Senator Charles Grassley (R- Iowa) objected to this arrangement and proposed an amendment that would have required congressional employees to be covered too. As described in Heritage Foundation Reports of October 23, 1989, this is what happened next: ''((The amendment)) sparked a heated debate in the Senate chamber, much of it unreported in the official Congressional Record . . . Grassley was surrounded by angry Democrats warning of the grave consequences should his amendment pass. One Senator, a chief sponsor of the bill, warned Grassley that his amendment would torpedo ((it)). Another Senator was even more candid in his off-the-record remarks, saying that the amendment would be enormously costly, forcing Congress to construct additional office buildings to accommodate all the new facilities required . . .'' This was a bill whose requirements were being sold to business as no big deal. Sequel to the story: Grassley stuck to his guns, forcing a vote on the issue. Ashamed to vote against his amendment, the Senate glumly passed it in a voice vote -- then sneakily eliminated it in conference. Ultimately, Grassley won a partial victory. He successfully attached an amendment to the 1991 Civil Rights Act specifying that ADA would apply to employees of the Senate. House employees are still uncovered. How big a deal is the ADA? FORTUNE has quoted regulatory specialists at the National Association of Manufacturers as worrying that it could be the most expensive (i.e., for business) legislation ever passed. Explaining what it entails for companies is a growth industry these days. Susan Lennox, a Boston attorney quite active in interpreting the law, was asked in an Industry Week interview how far employers must bend. Helpful answer: ''You have to bend over backward, but not so much that you hit your head on the floor.'' ADA bars discrimination (in employment, education, or access to public facilities) against the 43 million Americans deemed disabled. As we are reminded by that total, supplied by the Census Bureau and representing one of six Americans, the act has an expansive notion of what ''disabled'' means. Ramifying far from folks in wheelchairs, it includes individuals with low IQs, with most mental illnesses, and with a wide range of behavioral problems (e.g., drug addiction) that many voters think of as social pathology rather than disability. In any case, nobody knows how big the deal is because Congress, solely concerned with enveloping itself in an aura of lovability, neglected to define virtually all the crucial concepts in the act. The act says, for example, that employers must ''reasonably accommodate'' disabled job applicants. Does this mean that individuals unable to type, but interested in becoming legal secretaries, could require employers to buy software that activates voice- driven printers? Yes, it may mean that, or at least a recent advisory by consultant Jeanmarie Maher of San Francisco speaks sympathetically of this accommodation. The act also says that employers must not set job standards that are ''subterfuges'' used to screen out disabled applicants. Defying the dictionary, Congress specified that your standards may be judged subterfuges if they have the effect of screening out disabled people; your intent has nothing to do with it. More than two years after passage, the Equal Employment Opportunity Commission is still trying to define a subterfuge. The act also says employers need not accommodate applicants when doing so would represent ''undue hardship.'' What does that mean? A clue lies in one provision specifying, by way of example, that a large company could not cite undue hardship if it were asked to employ a companion for an otherwise qualified employee requiring help in job-related travels. The largest imponderable of all concerns the standards an employer may invoke in considering the qualifications of disabled people. The consultants now preparing business for the deadline keep saying that the act does not require anybody to hire an ''unqualified'' person. But employers do not divide the world into the qualified and the unqualified. They start out looking for the best available, and their definition of ''best'' incorporates a judgment about a worker's cost as well as ability. Since non-disabled people generally cost less, a rational employer would tend to prefer them, other things being equal. Unless of course the employer could hire the disabled person at a lower rate (which the act forbids). Are companies still allowed to prefer non- disabled workers on cost grounds? Or is that ''discrimination''? The answers to these questions could be a big deal.