The study of DNA damage at the chromosome level is an essential part of genetic toxicology because chromosomal mutation is an important event in carcinogenesis. The micronucleus assays have emerged as one of the preferred methods for assessing chromosome damage because they enable both chromosome loss and chromosome breakage to be measured reliably. Because micronuclei can only be expressed in cells that complete nuclear division a special method was developed that identifies such cells by their binucleate appearance when blocked from performing cytokinesis by cytochalasin-B, a microfilament-assembly inhibitor. The cytokinesis-block micronucleus (CBMN) assay allows better precision because the data obtained are not confounded by altered cell division kinetics caused by cytotoxicity of agents tested or suboptimal cell culture conditions. The method is now applied to various cell types for population monitoring of genetic damage, screening of chemicals for genotoxic potential, and for specific purposes such as prediction of the radiosensitivity of tumors and interindividual variation in radiosensitivity. In its current basic form the CBMN assay can provide, using simple morphological criteria, the following measures of genotoxicity and cytotoxicity: chromosome breakage, chromosome loss, chromosome rearrangement (nucleoplasmic bridges), cell division inhibition, necrosis, and apoptosis. The cytosine-arabinoside modification of the CBMN assay allows for measurement of excision-repairable lesions. The use of molecular probes enables chromosome loss to be distinguished from chromosome breakage and, importantly, nondisjunction in nonmicronucleated binucleated cells can be measured efficiently. The in vitro CBMN technique therefore provides multiple and complimentary measures of genotoxicity and cytotoxicity that can be achieved with relative ease within one system. The basic principles and methods (including detailed scoring criteria for all the genotoxicity and cytotoxicity end points) of the CBMN assay are described and areas for future development identified.