Variations in the C3 and C4 fractions of complement were studied in 15 patients who underwent surgery with either spinal anesthesia (5 cases) or general anesthesia using nitruos oxyde and a halogen compound (10 cases). Little peroperative variation was observed. An increase in C4 was observed in all cases postoperatively. Only a slight increase in C3 was observed after spinal anesthesia. The immune response of anesthetists exposed to halothane was investigated using PHA-induced lymphocyte stimulation (normal in males, elevated in females) and lymphocyte transformation to halothane and its metabolites (no evidence of allergy was observed).