The transformable mouse embryo fibroblast cell line C3H10T 1/2 C18 has been employed to study the induction by the carcinogen N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) of morphological transformation and mutation to ouabain resistance throughout the cell cycle. Cells were synchronized by means of isoleucine deprivation for 24 hr and initiated DNA synthesis with a high degree of synchrony 7.5 hr after release of the isoleucine block. At various intervals throughout the cell cycle cultures were treated with MNNG at 1.0 microgram/ml and the induction of cytotoxicity, morphological transformation, and ouabain-resistant colonies was determined. All three phenomena exhibited marked cell-cycle phase dependency. Maximal induction of transformation occurred in cultured treated 7.5 hr after release from isoleucine deprivation, when the cells were at the G1/S boundary. In contrast, induction of ouabain-resistant colonies was at a minimum at the time of maximal induction of transformation, and peak induction of ouabain resistance did not occur until 16-18 hr after release from the isoleucine block, when cells were in late S phase. A close correlation was observed between the induction of cytotoxicity and of ouabain-resistant mutants. The results suggest that differences exist in the production or cellular processing of the various early lesions.