The influence of serum natibodies and thermolabile serum factors on the intracellular killing of Staphylococcus aureus by neutrophil granulocytes has been examined using a method which facilitates a precise in vitro evaluation of the phagocytic and bactericidal activities of human polymorphonuclear leucocytes. The bactericidal activity of the granulocytes was significantly less pronounced in the presence of serum absorbed with Staph. aureus or inactivated at 56 degrees C for 30 minutes than in the presence of untreated serum. Specific antibodies seemed to stimulate the intracellular killing of bacteria more than thermolabile serum factors.