Micronucleus assays were carried out independently in two laboratories with 10 chemicals selected for aneuploidy studies in a coordinated project supported by the CEC. All chemicals were distributed from a single batch by the project coordinator. Male and female mice were treated by single i.p. injections, bone marrow was sampled 6, 24 and 48 h after treatment. Polychromatic and normochromatic erythrocytes were evaluated for the presence of micronuclei after May--Grünwald/Giemsa or Wright staining. The results from both laboratories are in close agreement even though different stocks of mice and different sample sizes were employed in the two laboratories. Only three of the 10 compounds induced micronuclei in mouse erythrocytes: colchicine, hydroquinone and vinblastine. The other seven chemicals gave negative results: econazole, chloralhydrate, diazepam, thiabendazole, cadmium chloride, thimerosal and pyrimethamine. The negative results for econazole and chloralhydrate were unexpected in light of the fact that these two chemicals were previously shown to increase the mitotic index and to induce c-mitotic effects in mouse bone marrow cells. The need for further investigations of these compounds by chromosome counting in second post-treatment metaphases is emphasized to determine their ability to induce aneuploidy.