Group B streptococci (GBS) are the most common cause of neonatal sepsis and pneumonia. The pathogenesis of GBS disease is not completely defined. GBS-induced endothelial cell injury is suggested by histological findings at autopsy and in animal studies. We hypothesized that (i) type III GBS (COH-1) invade and injure human umbilical vein endothelial (HUVE) cells and (ii) isogenic mutations in GBS capsule synthesis would influence HUVE invasion. Confluent HUVE monolayers were infected for 0.5, 2, or 6 h. Media with penicillin plus gentamicin were added and incubated for 2 h to kill extracellular bacteria. Cells were washed and lysed, and the number of live intracellular bacteria was determined by plate counting. COH-1 invaded HUVE cells in a time-dependent manner at levels 1,000-fold higher than those of the noninvasive Escherichia coli strain but significantly lower than those of Staphylococcus aureus. There was no evidence for net intracellular replication of GBS within HUVE cells. COH-1 infection of HUVE cells caused the release of lactate dehydrogenase activity. GBS invasion was inhibited by cytochalasin D in a dose-dependent manner; GBS-induced lactate dehydrogenase release was attenuated by cytochalasin D. The isogenic strains COH 1-11, devoid of capsular sialic acid, and COH 1-13, devoid of all type III capsule, invaded HUVE cells threeto fivefold more than the parent COH-1 strain. The type Ill capsular polysaccharide and particularly the capsular sialic acid attenuate GBS invasion of HUVE cells. Electron micrographs of lung tissue from a GBS-infected newborn Macaca nemestrina also showed GBS within capillary endothelial cells. We conclude that endothelial cell invasion and injury are potential mechanisms in the pathogenesis of GBS disease.