Unstimulated macrophages of X-irradiated mice have lost the capacity to induce an immune response to Shigella. Other studied functions of macrophages were unaffected or activated by X-irradiation. The phagocytic ability of irradiated macrophages, both in vivo and in vitro, was the same as that of normal cells. Irradiation of macrophage donors caused activation of several macrophage functions: DNA synthesis in irradiated macrophages was slightly higher, and RNA synthesis was six times higher than in non-irradiated controls. Choline uptake was significantly enhanced by irradiation. The levels of two lysosomal enzymes, acid phosphatase and cathepsin D, were elevated in macrophages taken from irradiated donors. The rate of particulate antigen (Shigella) degradation was slightly faster in irradiated macrophages, whereas no difference could be detected when soluble antigen ((T,G)-PRO--L) was employed.