A chemically induced, 20-methylcholanthrene, lymphoblastic sarcoma, in DBA/1J mice was used to determine the histocompatibility of the tumor between the DBA/1J and C57BL/GJ strains. Although 8 different histocompatibility sites exist between the 2 strains, this work demonstrated that these 8 sites were transmitted and expressed as a 5 unit discrete factor phenotypic variation. Fi x Fi hybrid animals were tested through the Fg generation and showed that only 5 linkage groups are involved in the transmission of the various histocompatibility sites. Survivors of the implanted F9 generation were used as progenitors for the Fi0 generation. Tumor take frequencies with the Fi0 generation demonstrated that the inheritance of tumor susceptibility behaved as a discrete function of a 5 unit inheritance factor.