Niridazole, an anthelminthic drug, has been shown to be a potent long-acting suppressant of cell-mediated immune responses both in man and in experimental animals. In the present study the effect of niridazole on the magnitude and kinetics of the primary and secondary antibody responses of mice to sheep erythrocytes, human serum albumin, and keyhold limpet hemocyanin was investigated. The therapeutic dose of niridazole for murine schistosomiasis moderately and transiently inhibited the primary antibody response to these antigens, but had no effect on ongoing antibody production against keyhole limpet hemocyanin or on secondary responses to any of the antigens. Single doses of niridazole (which suppress cell-mediated immune responses) had no effect whatsoever on the most strongly inhibited of the primary antibody responses, that to alum-precipitated polymerized human serum albumin.