The initial establishment of hamster long-term bone marrow (LTBM) cultures requires formation of an adherent stromal layer, but continued long-term proliferation of these cultures is best accomplished by removal of the suspension cells from the adherent layer and subsequent incubation in liquid suspension culture. Continued maintenance of bone marrow cells in the presence of the adherent layer for more than four to six weeks leads to a decline and eventual disappearance of erythroid and multipotent colony-forming cells. Addition of erythropoietin to LTBM suspension cultures produces mature, hemoglobinized erythrocytes. Incubation of the same cells plus erythropoietin in the presence of autologous parental adherent layers markedly inhibits both terminal erythroid differentiation and the number of detectable erythroid burst- forming units (BFU-Es). This erythroid inhibition occurs primarily within the first 24 hours with little or no effect on CFU-GEMMs or granulocyte-macrophage colony-forming units (GM-CFUs). However, continued incubation for seven days produces a reduction in all parameters. Removal of suspension cells from the adherent layer and restimulation with erythropoietin allows regeneration of erythropoiesis. Pretreatment of suspension cells with erythropoietin for 96 hours before exposure to the adherent culture only slightly inhibits erythropoiesis, suggesting that more mature erythroid progenitors are unaffected. Conditioned medium from the marrow adherent layer (ALCM) produces similar erythroid inhibitory effects in LTBM cultures with as little as two hours of incubation. The inhibition is actively produced by the adherent cells, since cycloheximide abolishes its production, while indomethacin has no apparent effect. Adherent marrow stromal cells may regulate hemopoiesis through negative as well as positive humoral signals, and they are particularly effective in erythroid regulation.