Transplants and primary cultures of the Harding Passey Melanoma (HPM) were found to consist of melanocytes and host macrophages. A pure population of melanocytes (obtained by subculturing) produced tumors when ip injected into Balb/c mice. There was a progressive infiltration of the tumors by peritoneal macrophages (PM). Repeated inoculations of irradiated HPM cells ip induced in vivo a protection against HPM graft. In vitro spleen cells produced a nonspecific inhibition of thymidine uptake by HPM. Normal PM were found to exhibit a spontaneous nonspecific inhibition. This capacity was impaired in PM taken from immune animals 7 days after immunization and recovered 31 days after. The relevance of such findings to cell mediated immunity and lysosome function are discussed.SCOPUS: ar.jinfo:eu-repo/semantics/publishe