Treatment of low density asynchronous cultures of C3H/10T1/2 Cl 8 mouse embryo fibroblasts with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) initiates the process of transformation and produces significant numbers of transformed foci only when treated cultures are subsequently exposed to the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). Cell culture variables which influence the outcome of this initiation and promotion system were studied. A TPA concentration of 0.25 micrograms/ml was found to be optimal for the promotion of focus production and the presence of TPA was required both during logarithmic growth and throughout confluence. The lot of fetal calf serum used to cultivate the cells also played a determining role in focus production. Of nine serum lots purchased from four different suppliers, only two were suited for initiation and promotion studies with MNNG and TPA. In contrast, seven of these lots were adequate for transformation studies with 3-methylcholanthrene. Factors which adversely influenced focus production included the use of fungizone or the use of high passage stock cultures. These studies demonstrate that cell culture variables which influence promotion in these cells can be controlled and that this system can be successfully used in studies of the cellular mechanism of in vitro promotion and for the detection of genotoxic substances.