In order to study the role of the T‐cell‐mediated immune defense in tumor development, a total of 93 sarcomas were induced using different doses (8 μg (0.1%), 40 μg (0.5%) and 400 μg (5%)) of 3‐methylcholanthrene in athymic nude Balb/c mice and phenotypically normal immunocompetent Balb/c mice. A shorter tumor induction time and a higher tumor incidence after treatment with low doses of methylcholanthrene were seen in nude mice than in immunocompetent mice, indicating that they have a lower resistance to the carcinogen. Contrary to expectations we found that the MHC class I expression of tumors from nude mice was lower than that of tumors from normal mice. Higher surface expression of MHC class I was demonstrated on high dose tumors from normal mice than on low dose tumors from normal mice. The cellular composition of the individual tumors raised in nude mice was more heterogeneous with respect to MHC class I expression. Since the mice differ genetically only with respect to the nu gene, these results indicate that a lack of T‐cell‐mediated defense mechanisms may confer upon the bearer a lower resistance to 3‐methylcholanthrene and a different MHC profile of the ensuing tumor.