Mice of strain G which have Fv-4rr genotype resemble those with Fv-2rr genotype in resisting the induction of spleen foci or splenomegaly by Friend leukemia virus. NB-tropic Friend virus replicates well in Fv-2rr mice, but not in G mice. To determine the Fv-2 genotype, G mice were crossed with Fv-2rr mice, and the hybrids were tested for susceptibility to NB-tropic Friend virus. The (G .times. Fv-2rr) F1 hybrids were similar to G. In F1 .times. Fv-2rr backcross generation, 3 types of mice appeared; (1) G type, (2) Fv-2rr type and (3) Fv-2ss type. Mice of the last type supported virus replication and developed splenomegaly. Mice of types 1, 2 and 3 appeared with the expected ratio of 2:1:1. In G mice, Fv-4r gene apparently suppresses the virus replication, and hence masks the expression of Fv-2s gene. Data of F2 generation support this conclusion.