C9 is required for rapid complement-mediated killing of E. coli. Previous studies revealed that 1) a diminished C9 concentration restricts the capacity of neonatal serum to kill E. coli and 2) pooled immunoglobulin G (IVIG) enhanced the capacity of human neonatal serum to deposit supplemental C9 onto E. coli and to kill the bacteria. Whether the administration of C9 potentiates the bactericidal and protective effect of IVIG in neonates infected with E. coli is not known. Therefore, in vitro and in vivo animal experiments were designed. Undegraded and hemolytically active human C9 was purified from Cohn Fraction III. IVIG was dialyzed against PBS without bacteriostatic agents. By rocket immunoelectrophoresis, the concentration of intrinsic C9 in serum pooled from two-day-old rats (NeoRS) was only 19% of adult controls. First, the survival of E. coli 07:K1:NM (5×104 cfu/ml) was quantitated in vitro during 60 min of incubation with 50% NeoRS. The bacteria proliferated in NeoRS (bacteria survival [±SEM]=170±14% of the original inoculum), and in NeoRS supplemented with 500 μg IVIG/ml(bacteria survival=122±13%). In NeoRS supplemented with C9 (100μg/ml), 79±14% of the bacteria survived. In contrast, only 30±10% of the bacteria survived in NeoRS supplemented with both IVIG and C9 (P<.04). Next, the survival of two-day-old rats was quantitated one wk after transthoracic injections of E. coli (5×103/g body wt [bw]). The bacteria were injected into each neonate 1 h after the intraperitoneal (ip) administration of IVIG (115 μg/g bw) or an equal volume of placebo (PBS). Each animal also received ip C9 (75 μg/g bw) or PBS 4 h prior to the bacterial injections. Of the 37 animals which received only placebo, none survived. 20 of 40 IVIG recipients and 20 of 38 C9 recipients survived. In contrast, 33 of the 40 neonates which received both IVIG and C9 survived (P<.01). Conclusion: Supplemental C9 potentiated 1) the bactericidal activity of IVIG in NeoRS and 2) the protective effect of IVIG in neonatal rats infected with an otherwise lethal dose of E. coli. These data may contribute to new strategies to reduce the morbidity and mortality of neonatal E. coli sepsis.Funded by the Alliant Community Trust Fund