In conjunction with 400 ng/ml murine monoclonal antibody of the IgM class, cord neutrophils were able to kill K1-positiveEscherichia coliin anin vitroopsonophagocytic assay. Neither adult nor cord neutrophils mediated bacterial killing in the absence of this monoclonal antibody. This bactericidal capacity was observed with eight of 11 (73%) but not all cord neutrophil samples, and under conditions of bacteria to neutrophil ratios as high as 15:1. Increasing the monoclonal antibody concentration up to 12 μg/ml paradoxically resulted in significantly lower amounts of bacterial killing. All 11 cord serum samples had sufficient complement activity to permit this monoclonal antibody to function; however, neonatal cerebrospinal fluid was not an effective complement source. We conclude that this monoclonal antibody may be a useful adjunct to conventional therapy of invasive disease caused by K1-positiveE. coli; but it would have to work at the bacteremic phase of infection.