Abstract Previous studies have indicated that cutaneous mast cells are involved in the elicitation of delayed-type hypersensitivity (DTH) in mice. Mast cells are thought to be required in DTH to release serotonin to open gaps between endothelial cells, allowing entrance of effector cells into the tissue. Two different strains of mice with independent genetic defects that lead to a substantial mast cell deficiency (W/Wv and SI/SId), and their normal littermate +/+ controls, were studied for their ability to express DTH. Both strains were shown to be deficient in serotonin-containing mast cells at skin sites of preferential elicitation of DTH in normal mice, such as the ear or footpad. Defective DTH was found in both mast cell-deficient strains by using two different systems: 1) sheep erythrocyte-induced footpad DTH, and 2) picryl chloride-induced contact sensitivity ear swelling responses. Adoptive transfer experiments demonstrated that abnormal DTH in mast cell-deficient mice was due to a defect in the elicitation of DTH, rather than a defect in the induction of effector T cells. In these experiments, the ability to elicit DTH could be transferred to normal +/+ mice with sensitized cells from mast cell-deficient mice, but sensitized cells from +/+ mice could not transfer DTH responsiveness to mast cell-deficient mice. In addition, no defects in numbers of epidermal Langerhans cells or in antigen-presenting cell function were found in W/Wv or SI/SId mice. We therefore concluded that abnormal elicitation of DTH in W/Wv and SI/SId mice was probably due to their mast cell deficiency. The inability of mast cell-deficient mice to express DTH was overcome when sensitized T cells and specific antigen were placed in the extravascular tissues by local passive transfer. These results suggest that mast cell release of vasoactive mediators, such as serotonin, is required in DTH to allow effector T cells to leave the intravascular space, enter the tissues, and become activated by antigen to release chemoattractant lymphokines that recruit a nonspecific infiltrate of inflammatory cells.