Lymphocyte subset analysis was performed on specific pathogen-free cats infected with feline immunodeficiency virus (FIV). As early as 4 weeks post-infection a sharp rise in lymphocytes expressing the feline CD8 marker (fCD8) occurred. No changes were observed in feline CD4+ (fCD4+) cell number throughout this period. The expanded subset displayed reduced expression of the fCD8 marker (fCD8low) compared to the control population (fCD8high). Dual-labelling revealed increased levels of major histocompatibility complex (MHC) class II antigens on the fCD8low subset compared to the fCD8high subset. The fCD8low population appeared to persist as it was detected in cats which had been infected for a period of 18 months and which displayed significantly reduced numbers of fCD4+ lymphocytes. The data suggest that infection with FIV induces rapid alterations in the lymphocyte profile of the cat characterized by the expansion of a fCD8+ lymphocyte subset. The persistence of this population throughout the course of infection suggests that the early events in FIV infection may be of importance in the pathogenesis of the disease.