A collaborative study between two laboratories has been carried out in order to investigate the reproducibility of the cytokinesis-block micronucleus assay in human lymphocytes. A modified protocol to obtain good quality slides from whole blood cultures has been developed. The spontaneous frequencies of micronuclei in binucleated lymphocytes from five male subjects were evaluated in each laboratory at two cytochalasin-B concentrations (3 and 6 μg/ml). Our data show that 6 μg/ml cytochalasin-B was more effective than 3 μg/ml to arrest cytokinesis in all the donors analysed. The mean frequency of micronuclei in binucleated cells was significantly lower at 6 μg/ml compared to 3 μg/ml cytochalasin-B in both laboratories. The results obtained indicate a very high concordance between both sets of data supporting the validity of the proposed protocol. © 1992 Oxford University Press.