We have found that in liquid cultures of spleen cells of adult Syrian hamsters of the F1D strain, the hematopoietic microenvironment is adequate to sustain proliferation of splenic stem cells for periods of greater than 4 mo, and permits granulocytic, monocytic, and megakaryocytic differentiation without secondary repopulation or addition of exogenous growth factors to the basic medium of RPMI 1640 plus 20% horse serum. Intimate topographical relations are established between spleen stromal cells and hematopoietic cell components of the culture is adherent "cell-producing" islets. Some of these islets are associated with multiple hematopoietic cell types such as myeloid, monocytic, and megakaryocytic cells. Other islets are associated with a single cell type such as megakaryocytes, which suggests a limited potential of some adherent stromal cells to direct the differentiation of precursor cells. Cultures of this type provide a simple and convenient model for investigation of the mechanisms controlling differentiation of hematopoietic stem cells, not only for granulocytic and monocytic cells, but for megakaryocytic cells as well.