Mutagenicity of fly ashes and bottom ashes of rice straw and rice husk was assayed by Ames' test. With respect to rice-straw ash, the extract from the fly ash was found to be more mutagenic than that from the bottom ash. In the case of rice husk, the mutagenicity of extract from the bottom ash was stronger than that from the fly ash. The extract from rice-husk bottom ash showed the strongest mutagenic activity among the four.