The basis for the observed mutagenicity of cyclohexanone oxime in the presence of hamster liver S9 in Salmonella typhimurium strain TA1535, but not in TA100, was explored. While the chemical had no effect on the appearance of the background lawn in either strain, it did cause a reduction in mutant colony counts in strain TA100, raising the possibility of selective toxicity to this strain. Viability of the two strains was determined directly by titering the cells in background lawns over a 3 day period. In order to do this, cells embedded in top agar overlays were released by extruding agar plugs through small holes in the bottoms of centrifuge tubes, followed by vigorous vortexing. Viable cell counts in background lawns of strain TA100, but not strain TA1535, were greatly reduced in the presence of cyclohexanone oxime. Most of the loss of viable TA100 cells occurred on days 2 and 3 following plating, after the cells had exhausted the histidine in the medium and stopped growing. Therefore, the observed loss of background lawn viable cells is unlikely to be the cause of the non-mutagenicity of cyclohexanone in strain TA100. Analysis of reversion spectra showed that cyclohexanone oxime-induced C→T transitions in the second position of the CCC triplet at the his mutation site in strain TA1535, but had no significant effect on any transition or transversion in strain TA100.