Radiolabeled mast cell activators, compound 48/80, and a cationic protein from neutrophils were shown to bind almost exclusively to mast cells in suspensions of rat peritoneal cells. A correlation was obtained between the binding of activator and release of 3H-serotonin at low concentrations of activator. It was shown that activator binding was substantially enhanced by the release process itself, thus demonstrating that release of amines exposes new sites for binding of activator. Saturation binding studies using compound 48/80 indicated that approximately 6×1010 molecules of activator could be bound per mast cell under conditions in which release of vasoactive amines occurs. A third mast cell activator, polymyxin B, was shown to compete with compound 48/80 for binding to mast cells. A technique has also been described for determining accurately the release of amines from mast cells. 3H-serotonin was incorporated in mast cells in vitro and specifically released by agents which stimulate the cells.