Toxic properties of the mycotoxins cyclopiazonic acid and aflatoxin B1 have been analyzed separately and in combination by monitoring their effects on luminescence in the marine bacterium Photobacterium phosphoreum, Strain NCMB 844. Genotoxicity was analyzed with a dark mutant of this organism whose reversion to the bioluminescent condition is stimulated by compounds attacking guanine sites in deoxyribonucleic acids. In this assay, cyclopiazonic acid, unlike aflatoxin B1, is not enhanced by cyclopiazonic acid when the two mycotoxins are assayed in combination. Cytotoxicity was assessed by the diminution of bioluminescence in a separate assay system with strain NRRLB-1177 of P. phosphoreum. Cyclopiazonic acid is more cytotoxic than aflatoxin B1, and concentrations of cyclopiazonic acid required for cytotoxicity decreases with time, whereas aflatoxin B1 cytotoxic expression does not change significantly with time under most assay conditions. Aflatoxin B1 and cyclopiazonic acid assayed as a dose pair indicate that these mycotoxins elicit their effects by independent modes of action.