Abstract Cell transformation assay using BALB/c 3T3 cells, C3H10T1/2 cells and others, can simulate the two-stage carcinogenesis utilized for formation of transformed foci. A sensitive cell transformation assay for tumor initiators as well as promoters has been developed using a v-Ha- ras-transfected BALB/c 3T3 cell line, Bhas 42; these cells are regarded as initiated in the two-stage paradigm of carcinogenesis. To distinguish between initiation and promotion, the initiation assay involves a 2-day treatment of low-density cells, obtained one day after plating, with a test chemical, and the promotion assay involves treatment of near-confluent cells with a test chemical for a period of 12 days (Day 3–14). When Bhas 42 cells were treated with tumor initiators, N-methyl- N’-nitro- N-nitrosoguanidine and 3-methylcholanthrene, transformed foci were induced in the initiation assay but not in the promotion assay. In contrast, tumor promoters, 12- O-tetradecanoylphorbol-13-acetate, lithocholic acid and okadaic acid, gave negative responses in the initiation assay but positive responses in the promotion assay. The results were reproducible with various treatment protocols. Sixteen polycyclic aromatic hydrocarbons were examined using both assays. Benzo[ a]pyrene and 7,12-dimethylbenz[ a]anthracene induced focus formation only in the initiation assay. Increase of focus formation was observed in the promotion assay with benzo[ e]pyrene, benzo[ ghi]perylene, 1-nitropyrene and pyrene. Benz[ a]anthracene, benz[ b]anthracene, chrysene and perylene showed positive responses in both initiation and promotion assays. Results of initiation and promotion assays of acenaphthylene, anthracene, coronene, 9,10-diphenylanthracene, naphthalene and phenanthrene were negative or equivocal. The present Bhas assays for the detection of either/both initiating and promoting activities of chemicals are sensitive and of high performance compared with other cell transformation assays.