Summary: The bactericidal activity of 20 maternal-neonatal paired sera was assessed employing a clinical type la, group B streptococcal isolate, strain 515, known to be opsonized by the classical complement pathway in a non-antibody dependent fashion. Twelve neonatal sera had efficient bactericidal activity for this isolate (mean 96%, range 91-99%) whereas eight had significantly lower bactericidal indices (mean 29%, range 0-54%) (p < 0.001). The mean bactericidal index for 20 maternal sera (88%) did not differ from that of 20 adult control sera (91%). Bactericidal activity was not influenced by the concentration of antibody to the capsular polysaccharide antigen of type Ia, group B Streptococcus present in these sera. When the classical pathway of selected neonatal sera with high or low bactericidal activity was inactivated by MgEGTA chelation of C1, bacterial growth was observed uniformly when concentrations of specific antibody were low. The bactericidal index of neonatal sera correlated significantly with the total hemolytic complement titer (r = 0.611, P < 0.01). The mean levels of each complement component or control protein assessed by radial immunodiffusion (C1q, C4, C3, B, H, and I) were lower in neonatal sera with low than in those with efficient bactericidal activity, and levels of C1q and H were depressed significantly (P < 0.025 and <0.001, respectively). Hemolytic titers of C4 but not C1 or C2 were also significantly lower for low than for selected high-killing neonatal sera (P < 0.05). Bactericidal activity in neonatal serum with a bactericidal index of zero was restored in a dose-dependent fashion by the addition of fresh frozen plasma.