The mutagenicity of dialkyl nitrosamines in the Salmonella plate assay

N-nitrosodimethylamine (DMN) is not mutagenic in the standard Salmonella plate incorporation assay (Ames test) in the presence of an in vitro metabolic activation system (S-9) derived from rat liver. When the S-9 was derived from Aroclor- or phenobarbital-induced mouse or hamster liver or from uninduced hamster liver, mutagenic activity was observed. Increasing the amount of S-9 above the usual maximum level of 50 microliter per plate increased the mutagenic response. Similarly, the mutagenicity of N-nitrosodiethylamine (DEN) and N-nitrosodi(n-butyl)amine (DBN) was greater in the presence of hamster liver S-9 than when mouse or rat liver was used. Data are also presented indicating that the ability of rat liver S-9 to mediate the mutagenic activity of DMN in the "preincubation" assay is due to the fact that the various components are present in this assay at several times the concentrations attained in the standard plate incorporation assay.