A validity test of the micro-forward mutation assay using Salmonella typhimurium strain TM677 was carried out using benzene-ethanol extracts from airborne particulates as test materials. Sensitivity of this assay in the presence and absence of S-9 mix was five to ten times higher than the pre-incubation method, a modified method of Ames's original method, using Salmonella typhimurium strains TA98 and TA100. This means that the virtual sensitivity of the micro-forward mutation assay is greater by ten times or more than the pre-incubation method, because two or more strains are necessary in the latter method. Repeatability of the micro-forward mutation assay was nearly same with that of the pre-incubation method. That is, the coefficient of variation of mutagenic frequency of the particulate extracts was about 10%. This forward mutation assay was applied to airborne particulates indoors, and proved to be able to measure easily the mutagenic activity of the extracts from particulates collected by 30 cu m-air sampling. Some indoor pollutants, especially ones collected in a room in which cigarette smoking was done, showed higher mutagenic activity than that of outdoor sample.