Mutagenic activity in particulate airborne pollutants in several samples collected in a wide variety of industrial, residential, and small-scale factory districts over the past seven years was detected by the Ames test. The particulate' air samples that were not contaminated with several chemicals such as NO (<0.001 ppm), NO2 (<0.001 pm), and SO2 (<0.001 ppm) showed only low mutagenic activity (1 revertant /m ) when they were tested with Salmonella typhimurium strain TA98 in the presence of S9 mix. However, most of the samples polluted by particulate matters showed high mutagenicity, with responses varying from 2.4 to 445 revertants per m3: 67 samples from an industrial area induced an average of 44 revertants per m3; 60 from a residential area, 16.2; and 10 from a small-scale factory area, 72.1. For assessing the mutagenic potential of the pollution in the atmosphere, the frequency of mutation determined with strain TA98 in the presence of S9 mix was used to divide the samples tentatively into five groups (A-E) on the basis of the normal logarithmic distribution curve of 137 samples. Air samples belonging to group A gave less than 2.3 revertants per m3 of air (1.12 f 0.12. no pollution); those of group B gave a range of 2.4 to 8.6 (5. 93 f 1.91, slight pollution); those of group C gave a range of 8.7 to 30.2 (16.0 f 5.36, moderate pollution); those of group D gave a range of 30.3 to I15 (56.7 f 20.1, considerable pollution); and those of group E gave more than 116 (234 f 119, heavy pollution). Of the 137 samples tested, 6 samples (4.4%) were assigned to group A, 38 (27.7%) to group B, 52 (38.0%) to group C, 34 (24.8%) to group D, and 7 (5.1 %) to group E. Furthermore, the samples in an industrial area were classified in the order of group C (35.8%), group B (26.9%), group D (22.4%), group E (8.96%). and group A (5.97%), and those in a residential area in the order of group C (46.7%), group B (33.3%), group D (18.3%), and group A (1.67%).