A monoclonal antibody (MCA) to enterotoxigenic Escherichia coli K99 antigen agglutinated K99+ enterotoxigenic E. coli strains B44 (O9:K30;K99;F41:H-) and B41 (O101:K99;F41:H-) grown at 37°C but not at 18°C. The MCA, which was characterized as immunoglobulin G1, reacted specifically with K99 antigen in an enzyme-linked immunosorbent assay and precipitated radiolabeled K99 antigen. A total of 45 colostrum-fed and colostrum-deprived calves were used in three separate trials to determine whether the orally administered K99-specific MCA would prevent diarrhea caused by strain B44. Twenty-eight calves were fed 1 ml of mouse ascitic fluid containing K99-specific MCA at 10 h of age and were orally challenged with strain B44 at 12 h to 14 h of age. Control calves either received no placebo or were fed 1 ml of mouse ascitic fluid containing fibronectin-specific MCA at 10 h of age. There was no difference in the incidence of diarrhea between the two groups after challenge. However, the severity of diarrhea, as evaluated by the proportion of calves in each group that developed severe dehydration, the degree of clinical dehydration, the degree of clinical depression, the degree of weight loss, and the duration of diarrhea after challenge was significantly reduced in calves that received the K99-specific MCA. The mortality rate was also significantly lower (P <0.001) in the treated (29%) than in the control (82%) group. These results suggest that orally administered K99-specific MCA can prevent severe fatal enteric colibacillosis.