The characteristics of cyclophosphamide-induced suppression of established ccll mediated immunity were studied in guinea pigs previously senstized to tuberculin. Cyclophosphamide treatment for 5 days produced a dose-dependent peripheral lymphoctopenia and disproportionatley greater neutrophenia which was particularly striking at high doses of 20 mg/kg per day(approximaetly 200 mg/kg-2 per day). Lymphoctes remianing in the circulation of cyclophosphamide treeated aniamls showed a doses-dependent reduction to both in vitro proliferactive and macrophage migration inhibitory factor responses to tuberculin compared to lymphocte responses of controls. Proliferative responses to phytohemaggultinin and concanavalin a were not significatly suppressed. Additional studies showed that cyclophosphamide suppressed the porliferactive and migration inhibitroy factor responses to tuberculin of lymph node and splenic as well as cirulating lymphocte populations. These studies showed that relatively short-term cyclophospamide administration produced immunosuppresion by quantitative as well as qualitative changes in lymphocyte populations. Significant suppresion of lymphocte function, howerver, was achived only with doses of cyclophoshamide which also produced a severe neutropenia.