Human marrow contains two interacting populations that affect granulopoiesis in culture. The first population consists of committed granulopoietic progenitors, while the second contains cells that produce factors whose activity is essential for granulopoiesis in culture. The two populations can be separated by an adherence procedure and assayed independently. Both populations proved to be heterogeneous: the granulopoietic progenitor pool contains a subpopulation which fails to respond to semipurified colony-stimulating activity derived from peripheral leukocytes but responds to the unpurified material. The population of "factor-producing" cells contains two subpopulations that can be separated on the basis of size by velocity sedimentation. Both produce factors that stimulate granulopoietic colony formation. However, the more slowly sedimenting subpopulation is capable of interacting with granulopoietic progenitors in suspension culture yielding an increase in their number, while cells from the more rapidly sedimenting population failed to support such an increase. The results are interpreted as indicating specificity in the interaction between granulopoietic progenitors and factor-producing cells.