A quantitative comparison was made between the induction of micronuclei and of chromosomal aberrations in human lymphocytes using a clastogen (adriamycin) and a non-disjunctional agent (vincristine) as model compounds. Cytochalasin B, an inhibitor of cytokinesis, was used to make it possible to evaluate the frequency of micronuclei only in cells that had divided after the treatment. The results showed that for the assessment of clastogenic damage due to adriamycin, the sensitivity of the micronucleus assay was comparable to that of conventional chromosomal analysis. For the non-disjunctional effect of vincristine, the micronucleus assay was much more sensitive than conventional chromosomal analysis.