This report presents the results of the U.S. Environmental Protection Agency's 1981 National Audit Program, which are summarized by pollutant and by analytical method. Semiannual audits were conducted for SO2 and NO2 (bubbler methods), Pb, NO3(negative 2) and SO4(negative 2) (filter strips) and CO (continuous monitors)2. One audit was conducted on high-volume flow rate. Continuous SO2 monitors were audited throughout the year, such that no monitor was audited more than once. Approximately 60 laboratories participated in each SO2 bubbler audit and 50 in each NO2 bubbler audit. Approximately 60 laboratories participated in each SO4(negative 2) audit, 50 in each NO3(negative 2) audit and 97 in each Pb audit. Approximately 380 CO monitors were checked in each CO audit and 159 continuous SO2 monitors were checked throughout the year. A total of 1,462 flow devices were audited. Approximately 6 percent of the overall results were rejected as statistical outliers. This value is consistent with that observed in previous audits.