(FORTUNE Magazine) – A MINI-EPIDEMIC of do-it-yourself medical-testing kits is about to break out. Thanks to the dramatic advances in biotechnology of the past decade or so, cheap and simple diagnostic tests for everything from pregnancy to strep throat are already starting to find their way into the home medicine cabinet. Industry analysts believe that by the early 1990s, when the home diagnostic tests now in the laboratory have made it to the marketplace, sales could reach $1 billion a year. This revolution in medical testing grows out of monoclonal antibody technology, pioneered by Cesar Milstein and George Kohler, whose work earned them the 1984 Nobel Prize for medicine. They found a way to clone pure antibodies, the proteins manufactured by the body's immune system when foreign substances, or antigens, intrude. By injecting a specific antigen into a mouse and then fusing the resulting antibodies with another type of cell called a myeloma, they produced hybrid cells that manufacture vast numbers of identical antibody cells. The antibodies can be used to detect such antigens as hormones secreted during pregnancy. Tests using monoclonal antibodies promise to be more trustworthy than older home diagnostic systems. The pregnancy tests, for example, are 95% reliable, vs. 75% for the premonoclonal variety. Other old-style home tests often produce either false negatives that keep patients from seeing a doctor when they should, or false positives that panic them. Home tests for cancer of the colon, for example, rely on detecting blood in the stool -- which can also indicate ulcers, hemorrhoids, or simply that the person tested ate raw meat in the past two days. Warner-Lambert, which offered the first home pregnancy test in 1977, has just introduced an improved version of ept-plus, an $11 kit that uses the new technology. Just one day after a woman has missed a menstrual period, she can mix a solution of monoclonal antibodies specific to the pregnancy hormone human chorionic gonadotropin into a urine sample. If the hormone is present, the antibodies bind to it and the solution changes from red to clear in ten minutes. Two similar kits -- First-Response Pregnancy, made by Tambrands (formerly Tampax) of Palmer, Massachusetts, and Advance, from Johnson & Johnson's Ortho Pharmaceuticals of Raritan, New Jersey -- employ sticks saturated with monoclonal antibodies. Tambrands' test can be used the first day of a missed period and gives a result in 20 minutes. OTHER TESTS are already here or in the works. For would-be parents, OvuStick, made by Monoclonal Antibodies Inc. of Mountain View, California (1984 sales: $5 million), offers $35 kits that determine a woman's peak fertility period. The Ames division of Miles Laboratories markets a $3 test for nitrites in urine, symptomatic of a urinary tract infection. Personal Diagnostics of Whippany, New Jersey (fiscal 1985 sales: $9.2 million), is working on tests to detect foreign substances like streptococci in saliva. Pharmaceutical giants are snapping up small, pioneering research firms or making deals with them. Eli Lilly paid $300 million in September for Hybritech of San Diego (1984 sales: $30 million). Bristol Myers laid out almost as much -- $294 million -- for Genetic Systems of Seattle, which had 1984 sales of $5.4 million. Ortho has a licensing agreement with Monoclonal Antibodies to produce Advance. More such hybrid fusions are likely. Explains Robert Peterson, a security analyst with R.C. Stamm of New York City, ''If you're a small company, you can build a better mousetrap, but you've got to have a sales force to sell it.''