A system for the cultivation of normal and malignant human bone marrow cells in liquid medium is described. The apparatus used is an in vitro diffusion chamber in which cells grow in suspension and upon a dialysis membrane. Proliferation and maturation of normal granulocytes and macrophages were sustained in culture for several weeks in the absence of exogenous stimulating substances. Cell replication was documented by a rise in viable counts, 3H-thymidine incorporation, and labeled mitoses. The entire maturational sequence of the granulocyte and macrophage series was identified in culture, and the cells were characterized by light and electron microscopy, cytochemical properties, phagocytic ability, and the presence of surface receptors for IgG. Nucleated red cell precursors were observed in some cultures for up to 11 days. Malignant cells from patients with various hematologic neoplasms were also successfully cultured. With this system human myeloma cells were maintained in culture for up to 3 wk on primary explant, and continued to synthesize immunoglobulin. The in vitro diffusion chamber technique permits the cultivation of normal human bone marrow cells in liquid medium and provides a convenient means for studying normal and neoplastic hematopoietic cell differentiation and function in short-term culture.