HLA class II–induced apoptosis is mediated via Fas and FasL interactions. Anti-Fas and anti-FasL antibodies significantly inhibited HLA class II–mediated apoptosis. 0.5 × 10⁶ splenic B lymphocytes were either left untreated for 96 hours or incubated for 96 hours with L227 XL (b) and for 48 hours with L227 XL with the addition of either 5 μg/mL anti-FasL antibody PE62 (c), 10 μg/mL antagonistic anti-Fas MoAb ZB4 (d), or 10 μg/mL control MoAb anti-CD19 (e) for a further 48 hours. 10,000 events were counted by FACS for each experiment.