A study was carried out on the function of macrophages in inducing antibody production to Shigella antigen, and on the effect of X-irradiation on the `immunogenic' function of macrophages. Peritoneal macrophages, which had been incubated with Shigella and then injected into mice exposed to 550 r, triggered the formation of agglutinating antibody in animals which did not respond to the injection of the antigen alone. The antibody formed was not produced by `contaminating' lymphocytes of the peritoneal exudate, since: (a) lymph node cells at doses higher than those of the macrophage inocula did not produce antibody when treated and injected under similar conditions, and (b) lymphocyte-free macrophage populations, obtained by culturing in vitro cells of peritoneal exudates, triggered the production of antibody when injected in to X-irradiated recipients after interaction with the antigen. Macrophages from irradiated donors incubated with Shigella were incapable of inducing antibody formation in X-irradiated mice. Animals exposed to higher doses of irradiation (900 r) did not produce antibody following injection of macrophage—antigen complexes. It was, therefore, concluded that macrophages from normal animals elicited the production of antibody by the lymphoid cells of the irradiated recipients.