In the coordinated programme to study aneuploidy induction, sponsored by the Commission of the European Communities, 10 known or suspect spindle poisons (colchicine, econazole, chioralhydrate, hydroquinone, diazepam, thiabendazole, cadmium chloride, pyrimethamine, vincristine and thimerosal) were tested in mouse bone marrow cells for the induction of c-mitotic effects. Three criteria were chosen: changes of the mitotic index, induction of chromatid contraction and spreading and decrease of anaphase frequencies. Among the chemicals tested colchicine, econazole, chloralhydrate, hydroquinone and vincristine were found positive. Diazepam, thiabendazole, cadmium chloride, pyrimethamine and thimerosal revealed no induction of c-mitotic effects under the conditions tested. Mitotic block and subsequent chromosome malsegregation (non-disjunction) are related phenomena. The three criteria chosen are considered as an indicative pre-screening test for the aneuploidy inducing potency of a chemical in mitotic, but not in meiotic cells. The present experiments were also regarded as a dose-finding exercise for further in vivo studies.