Feline immunodeficiency virus (FIV) infection in the cat is similar to human immunodeficiency virus type 1 infection in causing a selective reduction in CD4+ cell numbers, leading to inversion of the CD4+/CD8+ ratio. To determine whether FIV, similar to human immunodeficiency virus type 1, has a tropism for CD4+ cells, we examined the in vitro and in vivo susceptibilities of feline lymphocyte subpopulations to FIV infection. Infection of interleukin-2-dependent CD4+ or CD8+ lymphocyte cultures with the NCSU1 isolate of FIV (FIV-NCSU1) resulted in syncytium formation, cell death, and Mg(2+)-dependent reverse transcriptase (RT) activity in both cases. Monoclonal antibodies to feline lymphocyte subsets were used to sort peripheral blood mononuclear cells from FIV-infected cats into highly (> 95%) purified CD4+ cell, CD8+ cell, immunoglobulin-positive (Ig+) cell, and monocyte subpopulations. The mononuclear cell subpopulations were analyzed for FIV provirus by polymerase chain reaction and Southern blot analysis and for virus expression by RT activity. All 16 cats infected with FIV-NCSU1 demonstrated FIV provirus in CD4+ cell-, CD8+ cell-, and Ig+ cell-enriched lymphocyte populations. Southern blot detection of amplified gag gene sequences and limiting-cell-dilution polymerase chain reaction analysis indicated that Ig+ cells carried a higher FIV provirus burden in chronically (> or = 1-year) infected cats than either CD4+ or CD8+ cells. In contrast, CD4+ cells carried the greatest provirus burden in acutely (2- to 4-week) infected cats. FIV provirus was detected in monocytes from only 1 of 10 cats with asymptomatic infection. Addition of culture supernatants from enriched CD4+, CD8+, and Ig+ cells from FIV-infected cats to an FIV-susceptible CD4+ lymphocyte culture resulted in syncytium formation, cell death, and RT activity. Infection of Ig+ cells is not unique to FIV-NCSU1, as lymphocyte subpopulations from other cats with natural infections and cats infected with the Petaluma or Mount Airy isolate of FIV demonstrated a similar distribution of FIV provirus and RT activity. These data suggest that FIV possesses a broad tropism for peripheral blood mononuclear cells and that an Ig+ cell may serve as a major reservoir for the virus in chronically infected cats.