ALTHOUGH the complement-dependent bactericidal system of normal mammalian serum has been actively studied since the early descriptions of Nuttall1 and Buchner2, its role in native immunity is a matter of disagreement3. Recent reports indicate that strains of Gram-negative enteric bacilli isolated from patients' blood or from burns are ordinarily resistant to this system4,5. When equal numbers of a strain of E. coli resistant to this bactericidal effect and of one sensitive to it are injected into a rabbit, both strains ordinarily can be cultured from the blood stream 24 hr. following injection; but organisms from the resistant strain are 10–20 times more numerous6. There have been reports that strains of enteric bacilli resistant to the action of the complement-dependent system are more virulent for the mouse than are sensitive strains7,8. Such observations lead to the belief that bacterial strains sensitive to the action of this system are less likely to cause or maintain an infection than are resistant strains.