Hematopoietic marrow colonies were studied ultrastructurally to investigate stromal cell-hematopoietic cell interactions during early hematopoiesis. An elaborate interdigitative coupling of mature marrow macrophages to morphologically undifferentiated colony cells is described. This coupling was found in undifferentiated and granulocytic colonies and established a physical linkage of the macrophage and undifferentiated cell. The spaces between the coupled cells contained an electron-dense material and appeared to be modified by the uptake or release of cytoplasmic vesicles of the macrophage. Based on available evidence that hematopoietic stem cell proliferation is controlled at a local, or stromal, level, and the finding that this interaction occurs in very early hematopoietic colonies, it is suggested that interdigitative couplings represent a mechanism of stem cell regulation. In addition, the observations indicate macrophages to be actively involved with the lodgement of morphologically undifferentiated hematopoietic cells in the marrow space.