A semi-purified fraction extracted from fetal calf bone marrow was previously shown to contain a tetrapeptide N-Ac-Ser-Asp-Lys-Pro which inhibits in mice, hematopoietic stem cell entry into the cell cycle. This peptide however, did not exhibit any effect on either IL-3 nor GM-CSF dependent cell growth. We report herein that a semi-purified fraction also contains another activity which is antagonistic to IL-3. Addition of the fraction in vitro decreased IL-3 dependent mast cell colony formation. Growth of IL-3 dependent cell lines (DA-1 and FDC-P2) was also suppressed. No effect was observed in the same dose range on granulocyte-macrophage colony formation nor on IL-3 independent cell growth.