Mutagenicity studies were undertaken on lymphocytes from the following groups of persons exposed to styrene or polystyrene: one group from a styrene manufacturing plant, one from a polystyrene plant, and three from plants processing unsaturated polyester resins with different manufacturing methods. For all the groups the concentration of styrene in the atmosphere of the workplace was measured at various points and the amount of urinary mandelic acid was determined. In each case 100 metaphases were analyzed for chromosomal aberrations. The results were compared with reference groups.