The toxicology of chemicals used in the rubber industry is poorly known. In the present investigation an attempt was made to use biological monitoring methods to identify job categories with possible exposure to potentially genotoxic chemicals. Urine samples were collected and analyzed for mutagenic activity by the bacterial fluctuation test. The highest individual mutagenic activities were detected among the workers in the weighing and mixing departments, and some high individual values were found among vulcanizers. Sister chromatid exchanges (SCE) and structural chromosome aberrations were also analyzed from samples of peripheral blood. The effect of smoking was clearly associated with significantly increased numbers of SCEs and chromosome aberrations. Among the workers in different job categories, the highest SCE frequencies were found for mixers who smoked. Also nonsmoking cleaners of the mixing department differed significantly from the nonsmoking referents in this respect. However, only the group of nonsmoking weighers showed an increase in the number of chromosome aberrations (p less than 0.01) when compared with the referents. The results suggest the need for improved occupational hygiene in the weighing and mixing departments of rubber plants so that exposure to potentially hazardous genotoxic chemicals can be decreased.