(FORTUNE Magazine) – FORTUNE evaluated 130 of America's largest manufacturing companies before selecting the 30 featured in the three scorecards that follow. To determine ranking, we assigned values that range from zero (worst) to ten (best) to performance in 20 key areas. Not all carried equal weight, though. Among the categories given the most importance were the amount of a company's toxic chemical releases, adjusted for sales, and its percentage reduction of those releases; the comprehensiveness of a company's environmental program (whether , it has a written policy and goals, for example, or offers employee incentives); violations of environmental laws that carry large fines and penalties; and ratings by credible environmental groups. Other important categories, carrying slightly less weight, include whether a company is potentially responsible for cleaning up an inordinately large number of Superfund sites; whether it reuses and recycles hazardous and solid waste; and whether it participates in EPA's voluntary programs, such as 33-50 (so-called because it aims to reduce releases of 17 targeted chemicals 33% by 1992 and 50% by 1995 from a 1988 baseline). The main data source was the reams of information gathered by the U.S. government, particularly the EPA and the Occupational Safety and Health Administration (OSHA). Also helpful were the small but growing number of investment firms that analyze environmental performance, such as Franklin's Research and Development of Boston and Covenant Investment Management of Chicago. A major source of information was the Council on Economic Priorities (CEP), a respected New York City environmental group headed by Alice Tepper Marlin. CEP's corporate reports, supervised by senior researcher Kenneth P. Scott, are often used as references by the companies themselves. Most important, we attempted to interview, in person or by phone, all 130 companies surveyed. Only a handful refused to cooperate.