The in vitro survival of large human monocytes and the effect of ionizing radiation on the phagocytic function of these cells were determined 4 days following graded radiation doses. The monocyte survival curve was biphasic with a radiosensitive and a radioresistant population having DO [mean lethal dose] values of 55 and 650 rad and including about 25 and 75% of the cells, respectively. With regard to function, the ability of large monocytes to phagocytize latex particles did not appear to be significantly affected by ionizing radiation within the dose range studied. Monocytes are intrinsically more radioresistant than lymphocytes.