Eosinophils are closely associated with mast cells in immediate hypersensitivity reations. Activated eosinophils generate H2O2 and release granule components such as peroxidase (EPO) into the extracellular fluid. EPO forms a microbicidal and cytotoxic system when combined with H2O2 and a halide. EPO, which is a strongly basic protein, also binds firmly to the matrix of isolated mast cell granules (MC6) with retention and augmentation of peroxidatic activity. The cytotoxic effect of the MCG/EPO complex-H2O2-halide system will be assessed on various target cells such as tumor cells. The target cells will be labeled with 51Cr and the release of F1Cr from the cells will used to measure cytotoxicity of the peroxidase system. Mast cells also contain small amounts of endogenous peroxidase in their granules and the toxicity of the MCG peroxidase system on target cells will also be examined. The effect of hemeprotein inhibitors on the peroxidase system will be used to test for specificity of the reaction.