A study was conducted by the International Programme on Chemical Safety to examine variation between and within laboratories responsible for the extraction and the bioassay of complex environmental mixtures. Use of one or more complex mixtures from the U.S. National Institute of Standards and Technology (NIST) as a routine reference material in bioassay studies of complex environmental mixtures was considered. Diesel particles, air particles, and an extract of coal-tar (8007452) were bioassayed by 20 participating laboratories. Methodology involved use of the Salmonella/mammalian microsome plate incorporation test and strains (TA-98) and (TA-100). In all the laboratories, the particulate samples were mutagenic in both strains with and without activation. Mean extractable masses were 5 and 17.5% for the air and diesel particles, respectively. For (TA-100), with and without activation slope values for the air particulate were, respectively, 162 revertants per milligram (R/mg) and 137R/mg; for (TA-98), the respective diesel slope values were 268 and 269. The mutagenicity slope values for the diesel particles ranged from 3090R/mg for (TA- 98) with activation to 6697R/mg for (TA-100) with activation. For both strains, the coal-tar solution was negative without exogenous activation, but mutagenic with it. Benzo(a)pyrene (50328) and 1- nitropyrene (5522430), both positive controls, offered expected results.