Mutagenicity results are presented for 50 compounds tested in the mouse lymphoma TK+/(-)----TK-/- forward mutation assay. Test compounds were mostly from chemical classes not previously tested, to provide new information on the sensitivity of the assay; chemicals of low toxicity or thought to be non-carcinogenic and metabolic inhibitors, to indicate whether and under what conditions the assay can generate so-called false positive results. Twelve compounds that have been tested previously were included in this study to provide an indication of the reproducibility of the assay. Concordant results were obtained for nine of these, while disagreeing, positive results were seen with aniline, fluorene and pyrene. The following compounds belonging to the noncarcinogen category were positive at concentrations in the range 0.02-1 mol/l: dimethyl sulphoxide, EDTA, glucose, polyethyleneglycol, sodium chloride, sodium nitrite and urea. Measurements of the osmotic pressure indicated a lack of a simple relationship to mutagenic effects for these compounds. While the potent mutagenic/carcinogenic compounds tested gave greater than 4-fold increases in the mutation frequency, weak carcinogens or compounds not known to be carcinogenic that were positive in the assay gave increases of between 2- and 4-fold. Exceptions were aldehyde derivatives and chemicals that can lead to oxidative stress, which were detected with exaggerated sensitivity by the assay. Among the metabolic inhibitors tested, positive results were obtained with actinomycin D, cycloheximide, diethyl maleate, hydroxyurea and ouabain. Negative results were found with antimycin A. On the basis of the present results and previously published data it is concluded that a maximum limit for the test compound concentration can be set at 20 mmol/l and that testing to 20% total growth is adequate, with certain stipulations, to detect the mutagenic activity of test compounds. A similar analysis of the available test data shows that less than 4-fold increases in the mutation frequency have a lower predictivity for carcinogenicity.