The present study was designed to further evaluate the growth and progression of papillomas to squamous cell carcinomas (SCCs) in groups of animals receiving initiating doses of 7,12-dimethylbenz[a]anthracene (DMBA) producing relatively low papilloma yields following long term promotion (60 weeks) with 12-O-tetradecanoylphorbol-13-acetate (TPA). For comparison, groups of animals were initiated with various doses of DMBA and then promoted with mezerein (MEZ), benzoyl peroxide (BzPo) and chrysarobin (CHRY). Following initiation, groups of female SENCAR mice received the following promoter doses: TPA (1.0 or 2.0 micrograms per mouse); MEZ (2.0 micrograms per mouse); BzPo (20.0 mg per mouse); and CHRY (52.8 micrograms per mouse). The maximum papilloma to SCC conversion ratio obtained with TPA in the current study was 0.32. This value was in the range of maximum conversion ratios obtained with the other compounds: MEZ, 0.40; CHRY, 0.32 and BzPo, 0.19. In general, the highest papilloma to SCC conversion ratios observed with TPA as the promoter were obtained in groups that received the lowest doses of DMBA and had relatively low papilloma burdens. A comparison of papilloma to SCC conversion in groups of mice promoted with TPA, MEZ or CHRY and having similar papilloma yields, revealed very similar conversion ratios. Comparison of the BzPo group with a similar papilloma yield indicated that the conversion ratio was slightly lower with this promoter. The present results indicate that in mice promoted with TPA and having relatively low papilloma numbers, a larger proportion of these papillomas progress to SCCs during continued promoter treatment. Furthermore, the results suggest that papillomas behave similarly in their ability to progress to SCCs regardless of the promoter used when comparing groups of mice with similar tumor numbers. The data are discussed in terms of possible mechanisms for the observed results.