Feline retrovirus-induced possible preneoplastic lesions were identified by detection of feline oncovirus-associated cell membrane antigen (FOCMA) in bone marrow (BM) and mesenteric lymph nodes of noninbred, domestic short- or long-hair kittens. Cell-bound FOCMA was detected 1-3 weeks after feline leukemia virus (FeLV) infection and at least 6 months before a tumor was evident. BM FOCMA was detected earlier and reached higher levels at early periods in cats that became persistently viremic. Serum FeLV group-specific antigen (gsa) increased more rapidly and reached a higher level in cats that became persistently viremic, but nonviremic cats still had detectable gsa as late as 7-9 months after FeLV infection, even in the presence of high levels of viral antibody. The presence in nonviremic cats of FeLV gsa in sera and FOCMA in BM and sera suggests that these cats may have a persistent, low-level FeLV infection.