A collaborative study of chemically-induced mutagenicity was performed using the four bacterial strains Salmonella typhimurium TA102 and TA2638 and Escherichia coli WP2/pKM101 and WP2 uvrA/pKM101 in order to compare the specific spectrum of response to chemicals among the four strains and to determine the usefulness (sensitivity) of each strain. Twenty laboratories participated in this study. As the first step, 29 compounds were tested for mutagenicity using the plate incorporation method with or without metabolic activation. The compounds consisted of 12 chemicals judged previously positive only in E. coli WP2 uvrA/pKM101, 15 of their derivatives, and the 2 well-known mutagens hydrazine and formaldehyde. The strains and the chemicals were sent from a central source to each laboratory. The tests were performed in two laboratories per chemical. Concerning the result with each strain, the number of chemicals which showed mutagenic activity were 10, 7, 9 and 17 in TA102, TA2638, WP2/pKM101 and WP2 uvrA/pKM101, respectively. Among these 29 compounds tested, no qualitative difference in the response to chemicals among the four strains was observed with 17 compounds, being 12 negative chemicals and 5 positive chemicals. The remaining 12 compounds showed varying results among the four strains. On the comparison of TA102 and WP2 uvrA/pKM101, the same qualitative response to chemicals was observed with 22 compounds. Thus, although compounds tested in this study were selected, partly based on a previously-judged positive response only in E. coli WP2 uvrA/pKM101, 76% of test chemicals showed the same sensitivity in TA102, 7 chemicals (24%) were only positive in the E. coli strains. Of these 7 chemicals, 5 were the acrylic acid ester derivatives and the chloroacetic acid ester derivatives possessing a common structure of a functional group esterized between acid and alcohol with two or more carbon radicals.