Niridazole, a nitrothiazole derivative, used in the treatment of schistosomiasis, inhibits cell-mediated immunity in mice. The drug depressed delayed type hypersensitivity (DTH)3 3 Abbreviations used: CMI, cell-mediated immunity; DTH, delayed type hypersensitivity; FGG, fowl gamma globulin; KLH, keyhole limpet haemocyanin; POL, polymerized bacteria flagellin; NIP, 4-hydroxy-3-iodo-5-nitrophenylacetic acid; PFC, plaque-forming cells. when administered at the time of sensitization, and lymph node cells from niridazole-treated mice were found to be less efficient than cells from untreated controls in transferring DTH, or in providing helper function in a plaque-forming cell assay. In contrast, the drug was unable to prevent passive sensitization when given to the recipient of sensitized cells. These studies suggest that niridazole inhibits cell-mediated immune responses primarily by interfering with the afferent arm of T-cell sensitization.