The mathematical model used to calculate half-life and mutagenic concentrations of chemical carcinogens from the diffusion bioassay does not include any terms related to the nature of the microorganism used in the assay (Awerbuch et al., 1979; Awerbuch and Sinskey, 1980). In this work we tested the model with different strains of Salmonella typhimurium. These strains are auxotrophs for histidine and are sensitive to base-pair substitution. The half-life (tau 1/2) of N-methylN'-nitro-N-nitrosoguanidine (NG) was calculated by the diffusion assay, using strains hisG46, TA1950, TA1535 and TA100 as the bacterial indicators. For all strains tau 1/2 equalled 2.2 h; strain sensitivity for detecting threshold mutagenic concentrations of NG was essentially the same, except that hisG46 was slightly more sensitive.