One type of Th cell clone, Th1, causes delayed-type hypersensitivity (DTH) when injected with the appropriate Ag into the footpads of naive mice. Because IFN-gamma, one of the Th1-specific cytokines, has been reported to produce some of the manifestations of DTH, we have investigated the role of IFN-gamma in the DTH reaction induced by Th1 cells by using a mAb which neutralizes the biologic activity of IFN-gamma. Anti IFN-gamma inhibited up to 55% of the swelling response induced by seven Th1 clones in BALB/c and CBA/J mice suggesting that IFN-gamma is an important mediator of Th1-induced DTH. This inhibition was consistently observed during the peak phase of the DTH response and could be partly but not entirely explained by decreases in vascular leakage. The DTH responses induced by two Th1 clones in C57BL/6 mice were not inhibited by anti IFN-gamma. Taken together, these data suggest that other inflammatory mediators also play a role in Th1-induced DTH responses. Based on the gross description and kinetics of the response and the numbers of polymorphonuclear neutrophilic granulocytes in the cellular infiltrate, Th1-induced DTH appears to be similar to the Jones-Mote type of hypersensitivity.