Conditioned media containing colony-stimulating factors (CSF) were prepared in culture from unfractionated leukocytes and a lymphocyte-rich fraction of human peripheral blood. The mitogen phytohemagglutinin increased CSF production in low concentrations (106 cellsjml) of lymphocytes in the presence of homologous human plasma. In a semisolid agar medium system, the leukocyte-conditioned medium stimulated growth of granulocytic cell clones both from mouse and from human marrow cells; the lymphocyte-conditioned medium stimulated those from human marrow cells but had little or no activity with mouse marrow cells. This low colony-stimulating effect on mouse marrow cells was not due to the presence of inhibitors. The findings indicated that there were at least two types of CSF with respect to species specificity, that the lymphocyte-rich fraction contained CSF-producing cells, and that the factor(s) relatively specific for human cells could be separated from less specific factors.