Percentage of apoptotic B cells after 48 hours of culture in medium alone (control MEM) or in the presence of stromal cells (with or without contact). DNA fragmentation was revealed by flow cytometry. Results are expressed as mean ± SEM of 11 experiments for normal B cells and 20 for B-CLL cells. (□), Control MEM; (▨), stromal cell contact; (▪), no stromal cell contact.