The ability of sensitized rabbit leukocytes to release histamine from normal platelets is restricted to those cells which alone release histamine on addition of antigen. An excellent correlation between these two leukocyte functions has been obtained. Both reactions have similar metal and antigen requirements as well as parallel kinetics. In both cases cells from hyperimmunized animals are not responsive. Hapten specificity has been demonstrated for both the leukocyte and the leukocyte-dependent platelet response. Cell fractionation studies reinforced the conclusion that only those leukocytes which contain histamine and which release it on the addition of antigen can mediate this leukocyte-dependent platelet response. The lymphocytes, neutrophils and mononuclear cells derived from blood, tissues or exudates contain little or no histamine. Neither can these cells mediate the leukocyteplatelet histamine release. On these grounds it would seem that the sensitized basophil releases histamine on interaction with antigen and, through the mediation of a fluid phase intermediate, causes platelets to release histamine and leak intracellular 86Rb.