Human lymphocyte cultures were treated with increasing concentrations of diethyl sulfate (DES) at different times after stimulation with phytohemagglutinin (0, 24 and 45 h) and were scored for numerical chromosomal aberrations at different culture times (52, 72 and 96 h). A total of 3500 metaphases were analyzed. A statistically significant (p less than 0.01) increase in hypodiploid and hyperdiploid metaphases was found throughout the tested dose range (0.1-3 mM DES); the increase in polyploid metaphases was statistically significant at 1 mM (p less than 0.05) and 3 mM (p less than 0.01) DES. In human lymphocytes treated in Go, DES also induced chromatid breaks as well as micronuclei. In V79 Chinese hamster cells, DES induced micronuclei and polyploidy.