[en] Ouabain-resistant mutants were induced in C3H mouse embryo 10T1/2 fibroblasts by exposure to ultraviolet light, thus making available an in vitro system for studying mutagenesis and oncogenic transformation in parallel. ⁸⁶Rb uptake studies showed that biochemical mutants at the plasma membrane Na⁺, K⁺ transport ATPase (EC 3.6.1.3) locus were being selected for in this system. The optimal expression time for the mutants was found to depend on the dose of ultraviolet light, as was the induced mutation frequency. The ratio of transformation to mutation frequencies was found to be on the order of 10 for four different doses, suggesting that the target size in the cellular genome for transformation may be approximately 10 times the size of the Na⁺, K⁺ ATPase gene. We propose that both transformation and mutation induction can now be quantitatively studied in this single system