Although the Salmonella/plate test has been used extensively, a collaborative study was undertaken to determine the interlaboratory reproducibility of this microbial mutagenicity assay. Four laboratories participating in the study have completed testing, under code, of 61 carcinogens and noncarcinogens. All chemicals were tested both with and without metabolic activation in Salmonella typhimurium strains TA1535, 1537, 1538, 98, and 100. The metabolic activation systems used were derived from the livers of both uninduced and Aroclor 1254-induced Fischer rats, B6C3F1 mice, and Syrian hamsters. Analysis of the results on 23 of the chemicals tested in 3 of the participating laboratories showed that 8 were negative when tested in all laboratories and 13 were positive. Two chemicals gave positive results in 2 laboratories; the same 2 chemicals were negative when tested in the third laboratory.