There were fewer efficient phagocytes among leukocytes collected from artificially irritated mammary glands than among the leukocytes from blood of the same animals. The milk polymorphonuclear (PMN) leukocytes adhered poorly to a column of siliconised glass beads when compared with the blood cells. However, investigations of the O(2) uptake and CO(2) production of the milk PMN leukocytes revealed that these cells appeared to utilize metabolic pathways similar to those used by human and guinea pig PMN leukocytes during phagocytosis. These pathways are associated with degranulation and the production of H(2)O(2) following particle ingestion. It is therefore suggested that the milk PMN leukocytes appear not to have lost the ability to produce this bactericidal substance.