It has recently been shown that the classical 24-48-h component of delayed-type hypersensitivity (DTH) in mice is preceded by an early 2-h component. This early component of DTH resembles IgE-dependent hypersensitivity reactions, but is mediated by an antigen-specific T cell factor that activates cutaneous mast cells (MC). In the current study of cutaneous hypersensitivity reactions, evidence is presented that serotonin and histamine are released from the granules of MC in IgE-dependent responses, but that T cell activation of MC induces preferential release of serotonin. In T cell-dependent reactions, it was found that serotonin was released under experimental conditions in which MC granules were depleted of serotonin, but catabolism of serotonin in the cytosol was prevented. This suggests that T cells induce differential release of serotonin from non-granule storage sites in MC, such as transport vesicles in the cytoplasm. Morphological observations of MC in IgE-dependent compared to T cell-dependent cutaneous reactions were consistent with this hypothesis. Exocytosis of granules characterized IgE activation and was not found in MC that were activated by T cells, which instead showed mild degranulation accompanied by numerous cytoplasmic vesicles.