Ligation of the BCR does not prime B cells to HLA class I-mediated apoptosis and protects CD40-activated B cells. Naive B cells were cultured on irradiated CD32-L cells in the presence of 1 μg/mL anti-CD40 MoAb alone, 5 μg/mL anti-BCR MoAbs alone, or the combination of both and with 5 μg/mL control IgG1, W6/32, TP25.99, MoAb90, or 1 μg/mL anti-Fas MoAb CH11 added at the onset of the culture. Proliferation was measured by [³H]TdR incorporation after a pulse with [³H]TdR during the last 16 hours of the culture at the time indicated. Results are expressed as means ± SD of culture triplicates. [³H]TdR uptake of CD32-L cells was less than 500 cpm in the different experiments. (⋄) indicates IgG1, (○) indicates W6.32, (□) indicates TP25.99, (▿) indicates B9.12.1, (•) indicates MoAb90, (▪) indicates YTH 862, and (▴) indicates anti-Fas.