Partial restoration of the IL-6^-/- LTBMC hematopoietic support defect by exogenous IL-6. Wild-type and IL-6^-/- cultures were established and maintained as in Figure 1. Parallel IL-6^-/- cultures were maintained in 200 ng/mL IL-6, with weekly cytokine additions. Five weeks after initiation, cultures were analyzed for production of total hematopoietic cells (A), CFU-GMs (B), and BFU-Es (C) in culture medium and in the stromal layer. Results are expressed as the mean ± SD of 4 experiments. ^*P < .01, for wild-type versus IL-6^-/- cultures.