A study to identify putative bacterial pathogens in infants with necrotizing enterocolitis (NEC) was begun in 1976. Cultures of blood and of peritoneal fluid obtained by paracentesis were carried out in 25 infants with NEC. Segments of intestine excised at operation were Gram stained. Of the 25 infants, 8 recovered with medical management and 17 required operations. The 8 medically treated infants had sterile peritoneal fluid and, with 2 exceptions, sterile blood cultures. Of the 17 operated infants, 16 had bacteria in their blood and/or peritoneal fluid. The majority of resected bowel specimens from these infants contained a confirmatory morphologic type of bacterium within the wall. The clinical course of 8 infants with clostridia was compared to that of 8 infants with gram-negative enteric bacteria (Klebsiella, E. coli, or Bacteroides fragilis). The infants with clostridia were sicker. They had more extensive pneumatosis intestinalis, a higher incidence of portal venous gas, more rapid progression to gangrene, and more extensive gangrene. Infants with gram-negative rods had lower birth weights and lower platelet counts than the clostridial group. The difference in mortality between the two groups was not significant. The inherent pathogenicity of the gut flora may influence the clinical course of NEC. Among infants who develop intestinal gangrene, the clostridia appear to be more virulent than gram-negative enteric bacteria.