Animal experiments have indicated that styrene, which is a widely used organic solvent, may induce the microsomal enzyme function of the liver. Thirteen workers with long-term exposure to styrene in a polyester plant were investigated. They worked at air concentrations about the maximal allowed time-weighted average concentration of styrene in most Western countries (50 ppm). The clearance of antipyrine was determined from saliva concentrations before and after three weeks free of exposure and then again three weeks after returning to work. Thirteen matched controls were investigated with similar intervals and methods. No significant differences were found between the half life, apparent volume of distribution, or clearance of antipyrine either within the groups or between the groups. The data exclude (95% confidence limit) the possibility that occupational exposure to styrene at concentrations about 50 ppm stimulates the microsomal enzyme function of the liver to a degree compatible with an increase in antipyrine clearance of more than 2 ml x min-1 (3%). While the first antipyrine estimation was carried out under medical supervision, the workers themselves managed to perform the antipyrine test correctly after verbal and written instructions. This has broadened the application of the antipyrine test.