Peripheral blood lymphocytes (PBL) from cats infected with feline immunodeficiency virus (FIV) were examined for the occurrence of apoptosis after short-term culture. In the PBL from FIV-infected cats, changes in flow-cytometry scattergram, morphological characteristics of apoptosis and nucleosomal DNA fragmentation were observed. Percentages of apoptotic cells by flowcytometry analysis in PBL from FIV-infected cats (22.4% +/- 9.4%) were significantly higher than those in PBL from uninfected control cats (9.2% +/- 3.5%). The lymphocytes which underwent apoptosis included CD5+, CD4+ and sIgM+ cells, indicating that induction of apoptosis was not restricted to a special subset of lymphocytes. These findings provide evidence of the apoptotic state of PBL in cats with FIV infection.