The aneuploidy-inducing activity of extracts of diesel exhaust particulates from light duty (LD) and heavy duty (HD) engines was investigated in cultured peripheral blood lymphocytes of 8 healthy donors using the cytokinesis-block micronucleus test with the kinetochore labelling modification. A majority of the subjects tested showed a significant kinetochore-positive micronucleus induction after treatment with the highest dose (150 micrograms/ml) of LD extract, although some subjects also showed induction of kinetochore-negative micronuclei. Only one subject had significantly increased numbers of kinetochore-positive micronuclei at a dose of 400 micrograms/ml of HD extract. These results suggest that diesel extract, at least LD extract, possesses the ability to induce whole chromosome loss (aneuploidy) preferentially, although there are also chromosome breaks.