Soft agar colony formation of mouse epidermal cells during the early phase of two-stage chemically induced carcinogenesis.

A single topical application of 7,12-dimethylbenz[a]anthracene (DMBA) followed by twice-weekly 12-O-tetradecanoyl phorbol-13-acetate (TPA) treatment was given to the skin of female CD-1 mice, and soft agar colony formation of the epidermal cells was evaluated periodically during the treatment. From the second week, colony growth was observed and the number of colonies increased with time, reaching a significant level from the fourth week. Colony formation was observed earlier than papilloma development in the mouse skin. Soft agar colony-forming assay might be a useful tool to analyze the early events in carcinogenesis.