Intraperitoneal injection of slime glycolipoprotein (GLP) from Pseudomonas aeruginosa induced leukopenia and death of mice, similar to the effect of infection with viable organisms. Differential counts established that the leukopenia was characterized by a decrease in the number of polymorphonuclear leukocytes, followed by death of mice. Mice immunized with GLP survived challenge and responded with a leukocytosis that had a substantial increase in circulating polymorphonuclear leukocytes. Leukocytes from GLP-injected mice were agglutinated by anti-GLP serum, indicating an association between GLP and leukocytes. Other results indicated that 14C-labeled GLP is deposited mainly in the liver. Normal leukocytes labeled with 51Cr were injected intravenously into mice receiving an intraperitoneal injection of GLP. As with GLP, the 51Gr-labeled leukocytes were sequestered in the liver. These results indicate that GLP enters the blood stream and becomes associated mainly with neutrophils, and that the neutrophil-GLP complex is deposited in the liver, possibly accounting for the leukopenia in mice.