Mast cells were found to be unique among the peritoneal leukocytes by virtue of their capacity to enhance profoundly the proliferation of a variety of tumors in vitro. This phenomenon occurs at mast cell/tumor ratios which reflect the stoichiometry of host cell/tumor relationships in vivo. The growth factor was found to reside in mast cell granules and was identified as heparin by sequential purification and enzymatic degradation. This cellular interaction was tumor-specific, although isolated granules could enhance fibroblast proliferation. The findings are discussed in relation to previous morphologic studies, reports of in vitro mast-cell-mediated tumor cytotoxicity, and the role of mast cells in angiogenesis and connective tissue proliferation.