Mouse bone marrow cells cultured for 6 days in the presence of recombinant murine IL-3 and granulocyte-macrophage colony-stimulating factor (GM-CSF) were used as a source of precursors responsive to eosinopoietins. They were further cultured for 7 days in the presence of either a combination of recombinant cytokines or supernatants of bone marrow-derived mast cells (BMMC) activated with either immunological or nonimmunological stimuli. Cytosmears of collected cells were analyzed for eosinophil contents and allowed to demonstrate that supernatants of passively sensitized BMMC support both total cell proliferation and eosinophil production, after various periods of incubation with monoclonal rat anti-mouse IgE antibodies (the 6HD5 mAbs). In contrast, a stimulation with 100 ng/ml dinitrophenylated bovine serum albumin (DNP-BSA) did not generate supernatants displaying such bioactivities. Low doses of methyl ester of L (but not D)-leucine or of the calcium ionophore A23187 also allowed the release of eosinopoietic bioactivities. In addition, immunoreactive IL-5, GM-CSF, and IL-3 were quantified in the BMMC supernatants. These results demonstrate that activated BMMC are able to effect eosinophil production.