The effects of hypothermia and hemorrhage, alone and together, on reticuloendothelial system function have been studied in male BALB/c mice anesthetized with isoflurane. Whole body phagocytic activity was assessed at a deep body temperature (Tc) between 30 and 37 degrees C by measuring the clearance rate (K) of intravenously injected 51Cr-labeled sheep red blood cells (SRBC). There was a positive linear relationship between K and Tc. At a Tc of 30 degrees C splenic uptake of 51Cr-SRBC was reduced to approximately 50% of that at 37 degrees C, whilst liver and lung uptake were unaffected by the change in Tc. The hypothermia-induced reduction in K was rapidly reversed by rewarming to normothermia. A hemorrhage of 40% of measured blood volume followed after 30 min by return of the shed blood had no effect on K provided Tc was maintained at 37 degrees C. If Tc was allowed to fall to 30 degrees C during the hemorrhage, K was reduced to the same extent as in control hypothermic animals. There was also a tendency for uptake by liver, as well as spleen, to be reduced. These studies indicate that it is important to pay attention to core temperature when studying the effects of hemorrhage on aspects of reticuloendothelial function, at least in a small-animal model.