Previous studies using the in vitro diffusion chamber (Marbrook) have shown that bone marrow grown in this system will undergo limited stem cell replication and differentiation to mature granulocytes and mononuclear phagocytes. A series of studies with modified culture systems was initiated to improve cell production and committed stem cell (CFU-C) proliferation in vitro. Introduction of a continuous-flow system and a migration technique providing means of egress for mature neutrophils resulted in substantially improved performance. CFU-C were found to be capable of migration through a 3-mu pore membrane. These studies indicated that membrane surface area, culture medium circulation, and mature cell egress were among the conditions that could be optimized for maximum hematopoietic cell proliferation in suspension culture. The present observations also suggested that large- scale in vitro growth of mammalian bone marrow may be feasible.