We performed immunogold labeling with an ST-1 monoclonal antibody (IgM), specific for intact heparin, to define the subcellular localization of heparin in mast cells. Rat peritoneal mast cells were fixed by a modified Karnovsky method and embedded in Araldite. Ultrathin sections were first treated with sodium periodate and then sequentially incubated with MAb ST-1, rabbit anti-mouse IgM, and protein A-gold. By transmission electron microscopy, gold particles were localized inside cytoplasmic granules of peritoneal mast cells. In contrast, with the same procedure, no labeling was observed in mast cells from rat intestinal mucosa. Control sections of rat peritoneal or intestinal mucosa mast Mast cells cells treated with an irrelevant MAb (IgM) did not show any labeling. Treatment with nitrous Heparin acid abolished the reactivity of MAb ST-1 with peritoneal mast cells. These results Granules show that different mast cells can be identified regarding their heparin content by immunochemical procedures using MAb ST-1.