The objective of our study was to determine whether and how frequently MHC class I gene alterations occur in fibrosarcomas induced in BALB/c mice by the carcinogen 3-methylcholanthrene (3-MCA). Southern blot was performed to analyze the genomic organization of class I genes in a panel of twenty tumors of different immunogenic strength used at early in vivo passages. Our results rule out the presence of gross rearrangements or deletions in the H-2 class I genes of the tumors tested and indicate that the frequency of detectable class I alterations giving rise to restriction fragments length polymorphism (RFLP) should be very low.