The effects of micronucleus induction in mouse bone marrow cells by intraperitoneal injection (i.p.) and oral administration (p.o.) were studied with the spindle poison colchicine in 2 strains of mice (MS/Ae and CD-1). The final micronucleus test was performed with a 24-h sampling time at doses of 0.25-2.0 mg/kg by the i.p. route and 2-16 mg/kg by the p.o. route based on a simple acute toxicity test and a pilot micronucleus experiment. Colchicine induced micronuclei in both strains and at all doses tested. Micronuclei were induced more effectively by the i.p. route.