Intradermal administration of recombinant interferon gamma (rIFN-gamma) to lepromatous leprosy patients has converted the local histology toward a tuberculoid pattern. However, such changes have been confined to the site of injection. In contrast, in the present study, marked, intradermal accumulation of CD3+, CD4+, CD8+, and CD1a+ T cells and Leu-M5+ mononuclear phagocytes was induced at a distance from the sites of administration, in a dose-dependent manner, by 10 daily intramuscular injections of 10-30 micrograms rIFN-gamma/m2. Mononuclear cell infiltration began within 3 d of onset of rIFN-gamma therapy and persisted at least 8 wk. Intramuscular administration of rIFN-gamma to lepromatous patients receiving concurrent chemotherapy can safely induce widespread histologic features of an upgrading reaction.