Compliance with the Clean Air Act Amendments of 1970 by major air pollution sources can not be adequately determined due to ineffective oversight and inadequate recordkeeping by the Environmental Protection Agency (EPA). GAO found that: (1) in New York and Illinois, almost 70 percent of the sources subject to enforcement action since 1973 were not in compliance with emission limitations; and (2) in Illinois 321 major sources were not in final compliance at the end of fiscal year 1977. An enforcement action had never been taken against about one-half of these sources. EPA data systems, designed to track the amount of pollutants released into the air, are inaccurate and rarely compatible with the states' systems. As a result, incorrect progress and status reports have been issued to Congress and the public. EPA efforts to monitor and control air pollution have been ineffective because: (1) progress is measured by determining the number of sources, both complying with the standards and on compliance schedules, rather than considering emissions from all sources; (2) a source's compliance status is usually based on unverified information submitted by the source, rather than by more reliable methods, such as onsite testing or inspection; and (3) the EPA compliance monitoring program has shown that 22 percent of sources the states classified in compliance are actually not.