Three‐week‐old Big Blue™ (BB) B6C3F1 mice were given a single i.p. injection of ENU. Three weeks later, splenic T cells were isolated from each animal by ficoll gradient centrifugation and divided into two samples. One sample was cultured to measure hprt⁻ mutation and the other was used to extract DMA for lacl⁻ analysis. T cells from BB mice exposed to 0, 4.5, 13.5, and 40 mg ENU/kg (9 or 10 animals per group) displayed dose‐related increases in the frequency of both hprt⁻ and lacl⁻ mutations. Within each treatment group, the ENU‐induced mutation frequency (average observed mutation frequency minus average control frequency) was remarkably similar at the two loci. This suggests that treatments that increase mutation frequency at the endogenous hprt gene also produce similar incremental increases at the BB lacl transgene. However, because of the ten‐fold higher spontaneous mutation rate at lacl, the fold‐increase over background produced by ENU at this locus was significantly less than the fold‐increase produced at hprt. For example, the 4.5 mg ENU/kg treatment produced a 5.2‐fold increase above background at hprt (P = 0.001), whereas only a 1.5‐fold increase was produced at lacl (P = 0.140). Consequently, mutagenic insults that produce up to a fivefold increase in mutation frequency at an endogenous locus may be difficult to detect at the lacl transgene. Finally, the ENU‐induced response at hprt in BB mice was identical to that in generic B6C3F1 mice, suggesting that there are no inherent differences between transgenic and normal mice in their response to this mutagenic agent.

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