Abstract The present study examined whether anesthesia and operation could alter serum complement function as reflected by CH 50 levels and individual component activity. Classical complement pathway function was significantly reduced, as reflected by lowered CH 50 levels, in patients undergoing anesthesia and operation. This diminution in CH 50 levels progressed with time, reaching a maximum reduction of 25% when blood samples taken 2 hr postanesthesia induction were compared with preanesthesia control blood samples from the same patients. Future studies should demonstrate whether this mean reduction in CH 50 levels has any significant effect on host immune defense mechanisms in surgical patients. Results show that the greatest percentage reduction in complement component function involved C3 in both ASA Class I and II patients. This was followed in groups by alterations in C6 function, diminution in C5 in Class I and C4 in Class II patients. Perhaps the most significant result of this investigation is the demonstration that anesthesia and operation may induce a significant reduction in C3 function. This is especially important because of the role of C3 in both the classical and alternate pathways of complement activation.