Okadaic acid (OA) is a specific and strong inhibitor of protein phosphatases 1 and 2A present in eukaryotes, and a potent promoter of carcinogenesis in mouse skin. In this study, we examined the mutagenicity of OA. OA did not induce mutations in S. typhimurium TA100 and TA98, with or without a microsomal metabolic activation system. However, it was strongly mutagenic to Chinese hamster lung (CHL) cells without a microsomal activation system, as shown using diphtheria toxin (DT) resistance (DTr) as a selective marker. Treatment of CHL cells with OA at 17.5 ng/ml induced 164 DTr mutants per 10(6) survivors. A plot of the mutation frequency against the OA concentration gave a concave curve, and the mutant frequency was calculated to be 5500/10(6) survivors/micrograms, with OA in the dose range of 10-15 ng/ml. This value was about 680 times that of ethyl methanesulfonate (EMS), and comparable to that of 2-amino-N6-hydroxyadenine, one of the strongest known mutagens. Elongation factor 2 (EF-2) obtained from 4 DTr clones was not ADP-ribosylated by DT fragment A. PCR-direct sequencing revealed that the hot spot of EF-2 for EMS mutagenesis in CHO-K1 cells, the first letter of codon 717, was not a hot spot for OA mutagenesis in CHL cells.