Health surveillance for hazardous situations due to chemical exposure, in particular those which are carcinogenic, requires sensitive monitoring tests. Although experimental studies have shown the genotoxic and carcinogenic effect of several organochlorides, the lack of epidemiologic studies prevents their classification as carcinogenic to human beings. In this context, genotoxicity tests of short duration in human cells gain importance. The relation between the clastogenic effects (chromosome breaks) and cancer induction is already known to the scientific literature. The micronucleus test has been proposed as a good indicator of clastogenesis. In the present study, we evaluated, by means of the micronucleus test, 41 workers of a chemical industry in the state of Sao Paulo, southeast region of Brazil, who had been exposed to a mixture of chlorinated solvents (carbon tetrachloride, perchloroethylene, and hexachlorobenzene) and 28 workers who had not been exposed. Peripheral lymphocytes stimulated by phytohemagglutinin and with cytokinesis blocked by cytochalasin B were used. The results showed that the exposed workers presented a statistically significant higher frequency of micronuclei than the group which had not been exposed. (C) 1997 Wiley-Liss, Inc.2914652