The mutagenicity and benzo(g)pyrene (B~P) content of airborne particles (172 samples) from four locations in Athens was studied for 1 year starting Feburary 1984. All the organic extracts of airborne particulate matter showed direct mutagenic activity. There was a good correlation between B~P concentration and mutagenicity in all samples. Higher values of mutagenic activity and BctP concentrations were found at the sites located in the center of the city than at the sites located in the industrial areas. The height of the sampling place has a negative effect on both measured pollution parameters. The higher percentage of biological and chemical parameters were associated with particles < 3.3/zm in diameter. The mean yearly values of mutagenic activity of airborne particulates and BaP levels, were 1.9 rev m -3 and 2.6 ng m -3, respectively and can be grouped among the median, between the heavily and the lightly polluted cities of the world. Higher values of mutagenic activity were found during winter months. From the diurnal variation of mutagenic activity it was found that maximum value occurred during morning hours (0900-1100). The main source of mutagens seems to be the emissions from diesel powered engines and central heating.