The influence of T lymphocyte (T cell) subsets on the proliferation of the blood neutrophilic, macrophagic, and eosinophilic cluster and colony (aggregate) forming cells (CFC) was evaluated. The T cells and the CFC-enriched B-null cells (mononuclear cells depleted of monocytes and T cells) and null cells (B-null) cells depleted of B lymphocytes) were separated from the blood of normal individuals. The population enriched for OKT4 (T4 cells) monoclonal antibody-binding T helper/inducer cells and for OKT8 (T8 cells) monoclonal antibody- binding T suppressor/cytotoxic cells were separated from the T cells or mononuclear cells depleted of monocytes by negative enrichment using an immunoadherence “panning” method. These separated cell populations were cultured separately, and the B-null or null cells were cocultured with the T, T4, and T8 cells in agar medium in the presence and absence of an exogenous source of colony-stimulating activity (CSA). The cultures were evaluated at day 14. The B-null and null cells, but not the T cells or subsets, contained CFC. The number of CFC proliferated from the B-null or null cells increased significantly (p less than 0.001) in cocultures with unseparated T and T8 cells in a dose-dependent manner. The T4 cells neither promoted nor inhibited the CFC growth. The T8 population was a better promoter (p less than 0.01) of CFC growth than the unseparated T cells. This suggests that the CFC promoting effect of the unseparated T cells is probably due to the influence of the T8 subset present within the T cells. The proportion of the neutrophilic, macrophagic, and eosinophilic CFC that proliferated in these cultures was comparable under the influence of the T and T8 cells. The results of these studies indicate that the T8 population, which is the suppressor in the classical immune system, promotes the growth of the blood CFC. We speculate that the T cells are involved in the regulation of granulopoiesis in vivo.