Prior to administration, leukocyte transfusions are commonly irradiated with up to 5,000 R to eliminate lymphocytes and thereby prevent graft- versus-host disease in the recipient. It has been widely believed that phagocytes are resistant to this irradiation. In a recent report, we noted that phagocyte oxidative metabolism was compromised during preparation of white cells for transfusion. As part of our effort to examine the basis for this inhibition of phagocyte function during white cell preparation, we assessed the effects of irradiation on the long-lived monocytes that have been shown to persist at inflammatory foci posttransfusion. Human monocytes were irradiated for up to 3 min, receiving 2,500–5,000 R. This irradiation damaged human monocytes, significantly decreasing their in vitro survival for the first 3 wk of culture (p less than 0.02, irradiated versus control survival), and growth as assessed by two-dimensional cell size measurements during the first 2 wk of culture (p less than 0.01, irradiated versus control growth). Despite smaller cell size, total cell protein was significantly increased over time in irradiated cultures (p less than 0.001, irradiated versus control total protein per cell). Extracellular release of lysozyme and beta-glucuronidase per cell was not affected by irradiation, but extracellular lactate dehydrogenase (LDH) release was significantly increased after irradiation (p less than 0.001, irradiated versus control LDH release). Irradiated monocytes killed Listeria monocytogenes at a slower rate than the nonirradiated controls (p less than 0.05, irradiated versus control rate of killing). Thus, the data indicate that irradiation in doses used to prevent graft- versus-host disease in leukocyte transfusion recipients has a deleterious effect on in vitro human monocyte survival and function.