Serial measurements of CH50, C3, C4, and factor B were performed on three newborn infants with group B streptococcal sepsis. Two of the septic infants had a colonized but noninfected identical twin. All three infants with group B streptococcal sepsis had hypotension, prolonged coagulation times, neutropenia, and respiratory failure. During the course of the sepsis, factor B was depressed 30% to 35%, C3 was depressed 40% to 60%, and CH50 was depressed by 100% when compared to their cord blood levels. Two of the infants also had a 50% to 70% depression of C4. In contrast, no significant decrease in complement levels occurred in the siblings of the twins or in two additional control infants. These data are characteristic of older patients with Gram-negative sepsis and strongly suggest that the group B Streptococcus has endotoxin-like properties.