Abstract Carbaryl induced sister-chromatid exchanges (SCEs) but no thioguanine resistance in V79 Chinese hamster cells. Addition of S9 from Aroclor-pretreated rats, or glutathione, reduced the toxic effects of carbaryl. Glutathione or S9 mix reduced the effect of carbaryl on SCE. However, the latter result indicates that carbaryl's effect may be enhanced at a certain compound/S9 ration. Since treatment with microsomes alone, but not S9 mix, was clastogenic it cannot be excluded that this enhancement of SCE was due to perturbations in the S9 mix by carbaryl rather than to formation of some particular SCE-inducing metabolite from the compound. The effects of carbaryl on chromosomes and chromosomal distribution are comparable to those sometimes reported for TPA. This, in conjunction with the weak indications on carcinogenic activity of carbaryl, makes it of interest that the compound be tested for promotion or co-carcinogenicity in vivo.