The present study was performed within the project 'Genomic Mutations' (sponsored by the Commission of the European Communities) in order to gather all possible experimental information on 10 chemicals selected on the basis of their possible capacity to induce aneuploidy. An analysis of chromosomal aberrations was carried out in bone marrow cells of mice with the first five chemicals: colchicine (COL), econazole (EZ), chloralhydrate (CH), hydroquinone (HQ) and diazepam (DIAZ). The experiments were performed in parallel to micronucleus tests with the objective to distinguish a positive micronucleus response due to chromosomal breakage from that obtained by lagging chromosomes. The results of the micronucleus tests will be reported elsewhere. COL, CH, EZ and DIAZ showed no clastogenic effects in mouse bone marrow cells after single intraperitoneal injection. Polyploid cells were significantly more frequent after COL treatment. HQ showed a dose-dependent induction of chromosomal aberrations at 6 and 24 h after treatment. After 24 h, cells with multiple aberrations up to complete chromosome fragmentation were frequently observed. They indicate that a small fraction of the cell population, probably related to a specific stage of the cell cycle, was particularly sensitive to HQ. A sex difference in clastogenic response to HQ was not observed. It is concluded that of the five chemicals tested only HQ was clastogenic in mouse bone marrow cells under the present experimental conditions.