The chromosome aberration assay in vitro is a useful and sensitive test for detection of genotoxins. However, aberrations can occur secondary to toxicity, with compounds that do not react with DNA and are not genotoxic in vivo. Thus, some positive results in the in vitro aberration assay are not relevant to human risk. To help evaluate the influence of toxicity, data were collected from 27 pharmaceutical and chemical companies and contract laboratories. When cytotoxicity was measured by cell counts or confluence, compounds expected to damage DNA (Category 1) generally induced aberrations without severe concomitant cytotoxicity, i.e., at cell growth 60% or more of control. The more toxic nucleoside analogues, topoisomerase inhibitors, fluoroquinolone antibiotics, antifolates, and producers of reactive oxygen were still positive with cell growth 50% or more of control. In contrast, when there was evidence that the com-pounds were not DNA damaging (Category 2), there was a higher proportion of toxicity-associated clastogens, with positive results at less than 50% of control cell growth. When mitotic index (MI) was used as an indicator of cytotoxicity, the pattern was less clear, although there was a tendency to more mitotic suppression with the Category 2 compounds. Overall the data indicate that a limit on toxicity, and a more accurate way of estimating it, would increase the accuracy of the assay by reducing the frequency of nonrelevant positive results with a threshold-type of dose relation. The rationale for evaluating positive results in the in vitro aberration assay, especially those associated with toxicity, is discussed, as is the need for a harmonized regulatory approach.