Summary: Pursuant to a congressional request, GAO provided information on the Environmental Protection Agency's (EPA) progress in issuing maximum achievable control technology (MACT) standards required by the Clean Air Act Amendments of 1990. GAO noted that: (1) EPA has issued 26 of 45 MACT standards and plans to issue the other 19 MACT standards by its deadline; (2) EPA plans to meet the 1997 and 2000 deadlines for the remaining 129 standards; (3) EPA has had difficulty quantifying MACT standard benefits because of data limitations; (4) EPA has issued the hazardous organic national emission standards for hazardous air pollutants, the petroleum refinery standard, the coke battery standard, and the pulp, paper, and paperboard standard; and (5) although MACT standards are expected to have significant implementation costs and could cause certain price increases, facility closures, and job losses, the standards are expected to decrease air pollution and health problems, increase agricultural yields, and realize substantial savings.