Epidemiological studies have shown that those who work in rubber industry have an increased risk of cancer. In the working environment they are exposed, probably, to several hundred different chemicals some of them being known or suspected carcinogens and mutagens. The bacterial fluctuation test was used to detect the mutagenicity in the urines of exposed workers. A group of unexposed office clerks served as controls. Both groups consisted of smokers and non-smokers, and that was taken into consideration in the results. Rubber workers, either smokers or non-smokers, exhibited significantly higher mutagenic activity in their urine than the occupationally unexposed controls when the base-pair substitution strain E. coli WP2 uvrA was used as indicator organism. Use of the frameshift strain S. typhimurium TA98 revealed increased mutagenicity in the urine of occupationally exposed smokers, nonsmokers and unexposed smokers. The extent of mutagenicity in the urine of workers who smoked suggested a synergistic effect of smoking and occupational exposure. The bacterial fluctuation test with urine samples as sources of mutagenicity is able to detect chemical exposure if the excreted compounds are still in active form or can be activated. The method can be used to identify hazardous working conditions long before the manifestation of possible pathological changes in exposed individuals.