The ability of feline immunodeficiency virus (FIV) isolates from subtypes A and B to superinfect cats and cell cultures was tested. Three specific pathogen-free (SPF) cats were first inoculated with 10 ID50 of subtype B virus (FIVBang) and 30 weeks later inoculated with 100 ID50 of subtype A virus (FIVPet). On the basis of subtype-specific PCR analysis, both FIV subtypes were detected in the peripheral blood lymphocytes (PBLs) of two of three cats from 9 to 30 weeks following the second inoculation. Only the first virus was detected in the bone marrow (BM) cells of these two cats until 30 weeks following the second inoculation, at which time the second virus was finally detected in their BM cells. Both cats developed significant virus-neutralizing (VN) antibodies to the second virus by 15 weeks following the second inoculation; but only one cat had high VN titers to the first virus, which remained at the same level even after the second inoculation. The two control cats inoculated with only the second virus developed VN titers specifically to the second virus and were consistently PCR positive for the virus in PBLs and BM cells starting 9 weeks postinoculation. Thus a delay in BM infection with the second virus was observed in the two superinfected cats. In contrast, one of three cats had neither VN antibodies to the second virus nor PCR signal of the second virus in its PBLs, BM, and lymph node throughout the 30 weeks of study and it appeared to be resistant to superinfection.(ABSTRACT TRUNCATED AT 250 WORDS)