When mice were challenged intraperitoneally with 2.2 X 10(4) Pseudomonas aeruginosa in 0.5 ml of 2.7% mucin solution, the bacteria markedly increased in the peritoneal cavity, rapidly entered the blood stream, and the mice died, probably of bacteremia, within a short time. However, when mice were injected subcutaneously 4 hours prior to the challenge with 0.2 ml of 0.75% IgG (OEP-HA = 12.8) or IgM (OEP-HA = 64) separated from human plasma or 3% S-IgA EP-HA = 19.2) from fresh human milk, the bacteria were inhibited from propagating and were eliminated from the cavity so that they could not enter the blood stream. As a result, the mice tolerated the challenge and survived. When 3% IgA (OEP-HA less than 0.8) from human plasma was given, a complete cure was not effected in a few mice despite a general tendency of healing. Determination of the number of viable intra- and extracellular bacteria in the peritoneal cavity revealed that the infecting bacteria were rapidly phagocytized and killed by the peritoneal phagocytes in the presence of OEP-antibodies. These findings indicate that the effect of bacterial clearance from the cavity results from the cooperative action of OEP-antibodies and peritoneal phagocytes. Marked enhancement of phagocytosis by the antibodies was observed in an in vitro study in the presence of a heat-labile, complement-like substance(s). This finding is interpreted as evidence of the cooperative activity.