The carcinogenic activity of 5-methylchrysene (5-MeC) was assayed in the connective tissue of C-57 black mice, and 5-MeC was compared to benzo(a)pyrene (BaP) and certain modified chrysene derivatives for tumorigenic activity on mouse skin. Subcutaneous injections of 5-MeC (total dose, 0.5 mg) resulted in 22 sarcoma-bearing animals in a group of 25. On mouse skin, 5-MeC and BaP had comparable complete carcinogenic activity, and 5-MeC was a stronger tumor initiator than BaP. Compared to 5-MeC, 12-fluoro-5-methylchrysene was a weaker tumor initiator on mouse skin, while 5-methoxychrysene was somewhat less active. 1-Fluoro-4-methylchrysene and 6-methoxy-5-methylchrysene lacked significant tumor-initiating activity. The bioassay data are discussed in respect to the steric and electronic features of the chrysene ring system.