The bone-marrow micronucleus assay was used in mice to evaluate the mutagenic potential of cyanamide administered per os at doses of 10, 49 and 247 mg/kg. Bone-marrow smears were examined to find the incidence of micronucleated cells in 1000 polychromatic erythrocytes (PCE) and 1000 normochromatic erythrocytes ( NCE ). The ratio of NCE /PCE was also scored. No increase in the incidence of micronucleated polychromatic erythrocytes and no difference in the ratio NCE /PCE for the groups treated with cyanamide (10 and 49 mg/kg) were observed. The group treated with the highest dose of cyanamide (247 mg/kg) did, however, show an NCE /PCE ratio lower than the control group (p less than 0.05).