The photosynthetic flagellate Euglena gracilis contains enzymes which, like some P450 enzymes of human and rat liver, metabolize foreign compounds. Their metabolic activity was characterized in comparison to human and rat liver enzymes. The enzymatic oxidation of denitro-nifedipine, specific for human cytochrome P450 3A4, and the hydroxylation of (S)-warfarin, specific for human cytochrome P450 2C9, were both found in the flagellate by enzyme assays. In contrast, the hydroxylation of chlorzoxazone, specific for human cytochrome P450 2E1, and the oxygen-dependent cytochrome P450 2D6 specific demethylation of dextromethorphan, could not be detected. The substrates and chemicals used in the enzyme assays were tested for their toxic effect on Euglena gracilis with the biotest ECOTOX.