The nature of the suppressive activity in the peritoneal exudate cells (PEC) of guinea-pigs immunized with dinitrophenyl bovine gamma globulin (DNP50-BGG) was investigated. A method was developed to isolate from the peritoneal exudate large numbers of macrophages. Using density gradient centrifugation on Percoll it was possible to obtain a population of cells which contained over 90% macrophages. This macrophage preparation was found to respond to lymphokine but to be incapable of passively transferring delayed hypersensitivity reactions. When these immune macrophages were transferred into antigen immunized animals, which had been pretreated with cyclophosphamide (CY), the skin reactions were suppressed to the same extent as when the total PEC was transferred. PEC from guinea-pigs immunized with ovalbumin in Freund's incomplete adjuvant did not suppress the skin reactions in CY-pretreated DNP50BGG immunized animals. However, in contrast, macrophages from these animals did suppress the skin reactions in the recipient guinea-pigs indicating that the macrophage suppression was not antigen specific.