In the rice-producing district of Japan, environmental pollution by smoke from burnt rice straws has become a matter of concern. The mutagenicity of fly- and bottom-ashes of rice-straw and -husk was assayed by the Ames test, TA100 +/- S9 and TA98 +/- S9, and the relationship of combustion temperature to mutagenicity was investigated. Fly ash showed weak mutagenicity at 300 degrees C, with no remarkable change in mutagenic activity between 300 degrees C and 500 degrees C. Above 500 degrees C the mutagenic activity increased with a rise of temperature. The increasing rate of mutagenicity was much higher in the test system with S9 mix than that without S9. Moreover, the mutagenicity with TA100 was stronger than with TA98. With respect to bottom ash, weak mutagenic activity was observed at 300 degrees C, but at 400 degrees C decreased and at 500 degrees C or above could not be observed. Fly ash derived from burning 1 g of rice straw at 600 degrees C showed about 14 times and 2.5 times higher mutagenicity than main stream smoke condensates from the burning of 1 g of cigarette in TA100 + S9 and TA98 + S9 test systems respectively.