During feline leukemia virus (FeLV) infection in the domestic cat, viruses with a novel envelope gene arise by recombination between endogenous FeLV-related elements and the exogenous infecting species. These recombinant viruses (FeLV-B) are of uncertain disease association, but have been linked to the induction of thymic lymphoma. To assess the role of FeLV-B in the induction of multicentric lymphoma and other non-T-cell disease, the frequency of occurrence and nature of FeLV-B were examined in diseased tissues from a large collection of FeLV-infected animals. Diseased tissues were examined by Southern blot and PCR amplification to detect the presence of FeLV-B. Further analysis was performed to establish the recombination junctions and infectivity of FeLV-B in diseased tissues. The results confirmed the frequent association of FeLV-B with thymic lymphoma but showed infrequent generation, low levels and lack of infectivity of FeLV-B in non-T-cell diseases including multicentric lymphoma.