The protective value of antibody to the K1 capsular polysaccharide antigen of Escherichia coli was investigated in a newborn rat model of E. coli K1 infection. Pregnant rats were immunized intravenously with E. coli, and the agglutinating titer to meningococcal group B polysaccharide, which is identical to K1 polysaccharide, was measured in the serum of rats and their offspring. Convalescent serum from rat mothers showed an increased antibody titer in animals injected twice but not once with E. coli K1. Although no agglutinating antibody was detected in the serum of rat pups, animals suckled by mothers having a meningococcal group B agglutinating titer of 1:8 or greater had reduced infection and mortality rates after intraperitoneal injection with E. coli K1 compared with animals suckled by mothers having a low titer of agglutinating antibody (P < 0.05). In addition, greater protection could be conferred on rat sucklings by oral supplementation with a horse serum rich in antibody to meningococcal group B polysaccharide, suggesting that antibody was absorbed from the gastrointestinal tract and by itself could be protective. These studies demonstrated that antibody to the capsular polysaccharide of E. coli K1 altered the severity of E. coli K1 infection. Final clearance of bacteria from the blood appeared to await the maturation of other host defense systems in the newborn rat.