The ability of immature circulating neutrophils to phagocytose foreign particles was determined. One quarter of each of 4 cows, which were free of mastitis and pathogens and had milk somatic cell counts of less than 1 X 10(6) cells/ml, was infused with endotoxin of Escherichia coli (0.5 mg in 3 ml of saline solution) to induce a milk leukocytosis and a shift to the left in circulating leukocytes. Milk samples were taken from the infused and one uninfused quarter before and after infusion. Blood samples were collected at 9 hours after infusion, at 2-hour intervals from 13 to 21 hours, and at 36 hours after infusion. Phagocytosis was determined by incubating whole blood with yeast cells and determining the precentage of neutrophils phagocytosing and the number of yeast cells per neutrophil. Milk somatic cells increased from an average of less than 1.0 X 10(6) cells/ml before infusion to 37.0 X 10(6) cells/ml at the next milking (postinfusion hour 11). Circulating immature neutrophils increased from 7.6% before infusion to 56.6% at postinfusion hour 21. The percentage of neutrophils phagocytosing was 99 to 100 throughout the study. Number of yeast cells per neutrophil (mean phagocytosis) decreased from 6.5 before infusion was done to 5.2 after infusion. However, there was no correlation between percentage of immature neutrophils and mean phagocytosis. The decrease in yeast cells per neutrophil was attributed to the decrease in concentration of neutrophils and yeast cells in the in vitro procedure (r = 0.50). Apparently the immature neutrophils that appear after depletion of the mature form are as phagocytic as the mature neutrophils.