Chlorophyllin, a man-made water-soluble form of chlorophyll, is a focus of intensive studies from many laboratories for its antimutagenic and anticarcinogenic properties. Natural chlorophylls, in contrast, have been little studied in this regard. Since yellow-green vegetables are implicated to be protective against human cancers by epidemiological studies, it is important to explore the antigenotoxic properties of natural chlorophylls. Previously, we reported that a chlorophyll sample prepared from Chlorella vulgaris inhibited the mutagenicity of 3-hydroxyamino-1-methyl-5H-pyrido[4,3-b]indole, a direct-acting mutagen, in Salmonella, and that the chlorophyll also showed inhibition of wing spot formation in Drosophila induced by 3-amino-1-methyl-5H-pyrido[4,3-b]indole (Trp-P-2). We have now prepared several samples of chlorophyll from spinach and chlorella, and studied their effect on the genotoxicity of 4-nitroquinoline 1-oxide (4NQO) in Drosophila. The results showed that the genotoxicity of orally given 4NQO was suppressed by simultaneous administration of the chlorophylls. The mechanisms of this inhibition are discussed.