The effect of water-bath hyperthermia on rabbit peritoneal macrophages was studied in vitro. The cells were exposed to hyperthermia for 30 min to 4 hours and membrane transport of ions as measured by total and ouabain-inhibited 86Rb influx as well as membrane permeability for 86Rb and 51Cr-labelled intracellular proteins were investigated. Heat-treated macrophages were tested for their ability to phagocyte staphylococci and for reduction of nitroblue-tetrazolium. Moreover the effect of microwave whole-body hyperthermia on rabbit phogocytic cells was studied in vivo. Ion transport to macrophages was stimulated by both intensive (43°C) and moderate (40°C) hyperthermia. On the other hand exposition of the cells to 43°C led to pronounced release of g6Rb and "Cr from prelabelled cells. NBT reduction was generally decreased in macrophages exposed to 43°C and increased in macrophages kept at 40°C. Clearance of 32P-labelled staphylococci from peripheral blood of microwave-irradiated rabbits diminished when animals were exposed to microwave hyperthermia for 4 or 7 days (2 hours daily).