A highly sensitive ultramicro forward-mutation assay using Salmonella typhimurium TM677 was achieved by the introduction of a micro-vessel in the preincubation. The assay was about 10 times higher in sensitivity than the micro forward-mutation assay and was able to measure mutagenicity of extracts from airborne particulates obtained by only 3 cu m air sampling. Repeatability of the assay was nearly same as that of the micro forward-mutation assay, that is, the coefficient of variation of the mutation frequency for airborne particular extracts was 12.3% in the test condition without S9 mix and 13.7% with S9 mix. The assay was applied to the measurement of hourly variation mutagenic activity of airborne particulates indoors and outdoors, and revealed the mutagenic activity of indoor air was generally higher than that of outdoor air and that the mutagenic activity indoors in the absence of S9 mix was well correlated with human activity. It was also found by the assay and PAH analysis that indoor pollution by carcinogens and mutagens was largely affected by cigarette smoking and an air cleaner was useful for reduction of indoor pollution by mutagens and PAHs.