Fas ligation induces apoptotic cell death in naïve but not in memory CD4+ T cells. Naïve (A) or memory (B) cells were stimulated with 1 μg/ml of anti-CD3 antibody alone (dotted line) or with anti-CD3 plus 5 μg/ml of anti-CD28 (solid, thin line) or anti-Fas antibody (thick line) for 48 hr. Cells were permeabilized and stained with DNA-binding solutions and analyzed by flow cytometry. Apoptotic (<2n DNA) cells are those to the left of the gate. For naïve cells (A), 29% of cells were apoptotic (<2n DNA) when stimulated with anti-CD3 alone, 8% were apoptotic when treated with anti-CD3 and anti-CD28 antibodies, and 57% were apoptotic when treated with anti-CD3 and anti-Fas antibodies. For memory cells (B), percentages were 23, 12, and 18% for anti-CD3 alone, anti-CD3 + anti-CD28, and anti-CD3 + anti-Fas treatments, respectively.