In the framework of the coordinated programme 'Genomic Mutations' sponsored by the Commission of European Communities, eight known or suspected spindle poisons (cadmium chloride, chloral hydrate, colchicine, diazepam, econazole, hydroquinone, pyrimethamine, thiabendazole, thimerosal and vinblastine) were tested in a modified in vitro micronucleus test in Chinese hamster cells. Micronuclei (MN) with or without kinetochore were analyzed by staining of the kinetochore with an antikinetochore antibody (CREST staining). Mitotic index and ana-telophase:mitosis ratio were also recorded to evaluate cytotoxicity and c-mitotic effects of tested chemicals. CREST-positive MN were induced by diazepam, thiabendazole and vinblastine. Hydroquinone, pyrimethamine, econazole and cadmium chloride induced MN that contained both entire chromosomes and acentric fragments. Negative results were obtained with thimerosal. The results obtained indicate that the detection of CREST-positive MN is a powerful assay to identify spindle poisons. Furthermore, this method provides a useful tool to ascertain the origin(s) of induced MN.