Rabbit antisera (AMS) to guinea pig macrophages and the γG fraction thereof (AMG) were specific for membrane antigens of monocytes/macrophages. The AMS and AMG bound to the surface of peritoneal macrophages, agglutinated these cells, and destroyed them. They were nontoxic for guinea pig lymphocytes. When injected intraperitoneally repeatedly into Hartley guinea pigs immunized with BSA in complete Freund's adjuvant, AMG did not affect the Arthus and delayed hypersensitivity (DH) skin reactions elicited by BSA and PPD at 6 and 24 hr. They did, however, diminish or eliminate the passive transfer of DH in irradiated strain 13 guinea pigs that were infused with lymph node and a limited number of bone marrow cells from immune donors. This effect was most likely the result of a destructive action against bone marrow elements or their progeny.