An epithelioid cell line derived from fin tissue of bluegill sunfish (designated BG/F) exhibited early indications of cell transformation upon exposure to methylazoxymethanol acetate (MAM acetate). Such changes included the induction of polyploidy, increased colony-forming efficiency, loss of contact inhibition, and formation of transformed foci. Unlike later transformation characteristics observed with mammalian cells, the MAM acetate-treated BG/F cells could not be propagated under conditions of anchorage independence in soft agar. Incubation of BG/F cells with N-methyl-N'-nitro-N-nitrosoguanidine, followed by exposure to 12-O-tetradecanoylphorbol-13-acetate, was not observed to cause cell transformation under the experimental conditions. The controls of a fibroblastic cell culture derived from gill tissue of bluegill sunfish showed spontaneous transformation after 6 months of passage, similar to the transformation observed in the experimental MAM acetate treated gill cultures.