Murine hybridoma antibodies directed against the capsule and O-side chain determinants of theEscherichia colistrain Bort (018ac:Kl:H7) were evaluated for their ability to enhance bactericidal activity of cord blood against KlE. colistrains possessing O antigens common in neonatalE. coliinfection,i.e. 018, 07, and 01. The antibodies to the Kl capsule and O-side chain efficiently enhanced cord polymorphonuclear leukocyte-mediated killing of Kl encapsulatedE. colistrain possessing a homologous O antigen, but the IgM antibody to the Kl capsule exhibited approximately 10-fold greater activity than did the IgG3 antibody to O-side chain (weight basis). Both antibodies required complement for their opsonic activities. Our findings indicate that antibodies directed against the capsule and O-lipopolysaccharides are able restore the opsonic activity of cord blood against KlE. coli, suggesting that these antibodies may be useful in the prevention and therapy of neonatal.