Mice injected chronically with antiplatelet serum develop an increase in the number of megakaryocytic progenitor cells compared to animals given normal rabbit serum. To examine the specificity of this response, progenitor cells giving rise to megakaryocyte, granulocyte-macrophage, erythroid, and mixed-cell colonies were assayed after injection of various heterosera or saline. All four colony types increased in the serum-treated groups. Since the in vitro proliferation of hematopoietic progenitor cells is promoted by supernatants of mitogen-stimulated spleen cells, we hypothesized that the immune response following antiserum administration resulted in the in vivo activation of T lymphocytes which produced or led to the production of colony stimulating activities. To test this hypothesis, cyclosporin A, a preferential inhibitor of T lymphocyte function, was given to mice concurrently with antiserum and also added to spleen cell cultures in the presence of pokeweed mitogen. Cyclosporin A abrogated the antiserum- related increases in progenitor cell numbers in vivo and the production of colony stimulating activity in vitro. The results suggest that the immune response related to antiserum administration results in the in vivo production of hematopoietic colony stimulating activities that may be identical to those produced in vitro by mitogen-stimulation of spleen cells.