We have previously shown that the activity of the ionized mutagen, 1-hydroxmethylpyrene sulphate, is strongly enhanced in Salmonella typhimurium TA98, when KCl is present in the exposure medium (50-fold at a concentration of 125 mM KCl) and that the halogen ion is responsible for this effect. Here we show that KCl has the opposite effect on the activity of the lipophilic mutagen, 7-methylbenz[a]anthracene-5,6-oxide, (10-fold decrease at a concentration of 125 mM) and that K+ accounts for this influence. Many other solutes also decreased the mutagenicity of 7-methylbenz[a]anthracene-5,6-oxide, but to a smaller extent than the K+ salts. The stability of 7-methylbenz[a]anthracene-5,6-oxide did not appear to be altered in the presence of KCl (t1/2 approximately 12 min), as determined from mutagenicity experiments in which the test compound was added to the exposure medium at varying times before the bacteria. Furthermore, the influence of the exposure medium was significantly stronger in strain TA98 than in strain TA100. Taken together these findings argue for an influence of the medium on the bacteria rather than on the test compound. Parallel studies with other mutagens indicate that exposure in distilled water enhances the mutagenicity of many compounds. Exposure in distilled water, in combination with some other modifications, led to a 400-fold increase of the assay sensitivity towards 7-methylbenz[a]anthracene-5,6-oxide, as compared to the usual plate incorporation assay.