Two compounds found in garlic, diallyl sulfide (DAS) and diallyl disulfide (DDS), were tested for cytotoxic and genotoxic effects in a Chinese hamster ovary cell line. DDS was found to be more cytotoxic than DAS (showing a Dq of 1.6 micrograms/ml and a D0 of 0.6 microgram/ml as opposed to values of 295 and 90 micrograms/ml, respectively). Both compounds were found to induce both chromosome aberrations and sister chromatid exchanges (SCEs) with DDS again being more active on a weight-for-weight basis, exhibiting activity at concentrations below 10 micrograms/ml compared with the levels of 300 micrograms/ml and above required for DAS to show any effect. The addition of rat liver S-9 activation fraction to the assays modified the effects of the two compounds in a non-consistent manner. It reduced the induction of SCEs by both compounds, enhanced the generation of aberrations by DDS (but not by DAS) and radically altered the parameters of both survival curves, reducing the Dq values almost to zero but increasing the D0 values.