Supplementation of LTBMC-B of marrow from young and old mice with IL-7 increases the number of B-lineage cells produced per culture. The addition of rIL-7 at 0.5 ng/mL for 2 weeks increased the number of B-lineage cells produced in LTBMC-Bs of young and aged marrow. The total number of lymphoid cells per culture was determined using trypan blue exclusion and phase microscopy. The results were calculated from 2 to 5 pooled cultures. (▪) No added IL-7; () added IL-7.