Recent studies on neutrophils have revealed that these cells produce various cytokines, and may be involved in regulation of the immune response. We examined whether neutrophils are involved in delayed type hypersensitivity (DTH) to SRBC in rats by selective depletion of in vivo neutrophils using a mAb designated RP-3. When the rats had been treated with RP-3 at the time of priming with SRBC, DTH to these cells was inhibited. Furthermore, RP-3 treatment was effective in inhibiting the effector phase of the DTH response to SRBC. When spleen cells from rats that had been treated with RP-3 at the time of immunization were used for local transfer of DTH, footpad swelling was significantly less than that induced by spleen cells from the RP-3-untreated immune rats.