To elucidate the possible role of NF-kappaB in mouse skin carcinogenesis we studied the expression of p50 (NF-kappaB1), p52 (NF-kappaB2), p65 (RelA) and IkappaB-alpha inhibitor as well as kappaB-binding activity in adult SENCAR mouse skin, skin papillomas, and squamous cell carcinomas (SCC) generated by a two-stage carcinogenesis protocol. We found that in normal epidermis all of the above proteins were mostly expressed in the cytoplasm of basal cells. Western blot analysis revealed a dramatic increase of p50 and p52 expression in mouse skin tumors starting from the middle stage of promotion. We also found that the level of IkappaB-alpha protein in many late papillomas and SCC was lower than in normal epidermis. Results of EMSA showed an increase in kappaB-binding activity in mouse skin tumors and suggested that p50 is the major component of constitutive kappaB-binding complexes in normal epidermis and in tumors. It has been shown that nuclear IkappaB protein Bcl-3 is able to increase p50/p50 homodimer binding to the different kappaB sites in mouse thymocytes. Our finding on Bcl-3 overexpression in late papillomas and SCC could explain the selective increase of p50-related kappaB-binding in mouse skin tumors. Thus, our results strongly suggest the important role of p50 in skin carcinogenesis.