In a comparative study on 135 compounds of various chemical classes, 30 agents inducing direct nonreparable DNA damage in repair-deficient E. coli failed in reverting strains TA1535, TA1537, TA1538, TA98 and TA100 of S. typhimurium (De Flora et al., 1984b). These compounds were re-assayed in the Ames test using strains TA97 and TA102. A dose-dependent mutagenic response was detected with aminoantipyrine and p-rosaniline in TA97 and with streptomycin and formaldehyde in TA102. p-Rosaniline was the only mutagen requiring metabolic activation. 5 compounds, i.e. o-aminophenol in TA97 and methanol, ethanol, cadmium chloride and cadmium sulfate in TA102, induced a reproducible increase in revertants over controls, but this was less than 2-fold. The remaining 21 chemicals--including amino compounds, aliphatics, aromatics, heterocycles, hydrazine derivatives and inorganics--confirmed their inactivity in the Ames test. Overall data for 135 compounds, comparing the Ames test (7 strains) and the DNA-repair test (3 strains), are re-assessed on the basis of these findings.