B-lymphocyte line lacking Fas is not susceptible to HLA class II–mediated death. 0.5 × 10⁶ cells were incubated for 24 hours (a, RAJI; c, Fas⁻) or 48 hours (b, RAJI; d, Fas⁻) with L227 XL before quantifying the apoptotic nuclei. EBV-transformed Burkitt's lymphoma cell line RAJI was compared with the EBV-transformed Fas⁻ cells. The background level of apoptotic nuclei in nonstimulated cells was 2.5% for RAJI and 3.7% for Fas⁻. Insets showing the morphology typical of apoptosis depict either Hoescht 33342–stained RAJI incubated for 48 hours with L227 (b) or Hoescht 33342–stained Fas⁻ incubated for 24 hours with 50 ng/mL staurosporine (d).