The concentrations of mandelic and phenylglyoxylic acids, urinary metabolites of styrene, and styrene in blood were determined for 491 styrene polymerization workers. Styrene in subcutaneous fat was determined for 25 workers. The levels of styrene exposure were estimated to be less than 10 ppm, and urinary metabolite and blood styrene concentrations indicated that significant recent exposure (within 4 h) had occurred among workers in areas of styrene polymerization and styrene monomer production. Styrene analysis of subcutaneous fat suggested that the monomer may have been present for as long as 3 d after exposure, a time when urinary metabolites and blood styrene were almost invariably undetectable. All three biological parameters were correlated with recency of exposure and estimated intensity of exposure within job categories.