Effect of stroma CM on B cell apoptosis. Stroma CM was prepared as described in Materials and Methods. Results are expressed as the mean ± SEM of nine experiments and represent the percentage of apoptotic B cells after 48 hours of culture in the presence of α-MEM supplemented with stroma CM at 50% compared with cultures in α-MEM alone or in the presence of stromal cells but without contact. (□), Control MEM; (▨), stroma CM; (▪), no stromal cell contact.