Stanziale, W. G. (Fort Detrick, Frederick, Md.) and J. D. White. Influence of guinea pig plasma factors on phagocytosis of Pasteurella pestis. I. Plasma from normal guinea pigs. J. Bacteriol. 83:175-181. 1962.-The existence of an enhancing effect of normal guinea pig plasma on the phagocytosis of Pasteurella pestis has been reaffirmed, using a test system consisting of neutrophils from normal nonimmune guinea pigs as the phagocytes. This enhancing influence was found to be attributable to the interplay of a thermolabile and a thermostable plasma component. The data suggest a consideration of complement as the thermolabile component, since the enhancing activity of plasma is destroyed by heating to 56 C and restored by addition of fresh plasma. The possibility that the thermolabile plasma component is opsonin was discounted. Surface alterations which did occur resulted in preserving the integrity of the envelope substance of the plague bacillus. Treatment of neutrophils with plasma prior to phagocytosis did not increase their phagocytic capacity. The thermostable enhancing component was not demonstrated to be present in any of three purified serum protein fractions tested. In fact, all of these fractions inhibited phagocytosis of P. pestis. An enhancing effect of purified fibrinogen could not be demonstrated.