In this report, the biological properties of human recombinant interleukin-3 (rhIL-3) were studied. We investigated the range of unfractionated, purified and single cell human progenitors responsive to IL-3; compared the colony types observed with those obtained in the presence of recombinant human granulocyte-macrophage colony-stimulating factor (GM-CSF) or granulocyte-CSF (G-CSF). The results show that IL-3 directly stimulates the formation of colonies derived from eosinophil and, to a lesser degree, granulocyte and macrophage progenitors. In combination with erythropoietin, it supports the development of erythroid and mixed-erythroid colonies. Furthermore, the data show that IL-3 is a more potent stimulus for both erythroid and eosinophil progenitors than GM-CSF. Interleukin-3 stimulates the formation of both compact and dispersed colonies derived from eosinophil progenitors, whereas GM-CSF stimulates the formation of only the compact type. We conclude that some of the proliferative effects of IL-3 observed on unfractionated and semipurified bone marrow populations are indirect and most likely involve accessory cell interactions.