Ingestion of bovine red blood cells opsonized with IgG, by irradiated and control cultures of mouse peritoneal macrophages, was monitored at various times following exposure to 7.5-20 Gy of 60Co. Radiation produced decreases in the percentage of phagocytic cells and reduced the phagocytic index of the macrophages at 6-10 days post-irradiation. Only a small decrease in the phagocytic index of irradiated cultures was noted on day 3 post-irradiation. Cell survival as monitored by cell number and lactic dehydrogenase release as well as the levels of beta-glucuronidase and lysozyme were less sensitive to radiation exposure than was the phagocytic ability of the cultures. Addition of 8-bromo-3',5'-cyclic adenosine monophosphate and prostaglandin E2 to cultures increased the phagocytic ability of both irradiated and control cultures but did not abolish the deficit produced by radiation. The data indicate that in vitro radiation exposure produces time-dependent changes in the ability of mouse peritoneal cells to ingest IgG coated red blood cells.