Previous studies on several samples of urban air-borne particulates showed that the long-chain fatty acids present in these samples can interfere with the measurement of mutagenicity of the particulates with the Salmonella assay. To explore whether this phenomenon is a general, fatty acid contents and the mutagenicity (with Salmonella typhimurium TA98 without S9) were measured for 34 particulate samples collected in the cities of Okayama and Tokyo over a period of 1 year. Palmitic, stearic, oleic and linoleic acids were found in all these samples in this order of amount, and their interference on mutagenicity measurement was eminent, particularly at high doses of the sample. With the use of blue cotton extraction, the mutagenic components can be freed from most of these antimutagenic factors. Significant correlation was found between the number of particulates and the mutagenicity per unit volume of the air. Eight polycyclic aromatic hydrocarbon compounds, including benzo[alpha]pyrene were quantified for these 34 particulate samples. Their contents were too small to account for the observed mutagenicity, suggesting that other polycyclic compounds, possibly involving nitro aromatics, were responsible for the mutagenicity observed. No remarkable differences were noted between Okayama and Tokyo in fatty acid contents, mutagenicity or polycyclic aromatic-hydrocarbon contents of the samples.