The fate of Escherichia coli strains within the polymorphonuclear leukocytes was studied by determining the killing of bacteria, measuring the release of degradation products, and examining the phagocytic bacteria by electron microscopy. When sufficiently opsonized, both unencapsulated and encapsulated E. coli strains were rapidly phagocytized by polymorphonuclear leukocytes. Once phagocytized, the two unencapsulated E. coli strains (K-12 and O111) were rapidly killed (99% of the bacteria were killed during the first 5 min of phagocytosis) and extensively degraded (about 40% of the radiolabeled material was released from bacteria after 15 min of phagocytosis). Electron micrographs taken after 15 min of phagocytosis revealed extensive structural changes in most of the internalized bacteria. In contrast to the rapid killing and extensive breakdown of these strains, encapsulated E. coli O78:K80 was more resistant to killing and withstood degradation by polymorphonuclear leukocytes (only 5% of the radioactivity was released from the radiolabeled bacteria after 1 h of phagocytosis). Electron micrographs of thin sections taken after 1 h of phagocytosis revealed virtually no structural changes. Most of the internalized bacteria were still surrounded by thick capsular material.