Anticolibacillus antibody activity was examined in 16 samples of bovine colostrum by O-agglutination test with 6 serotypes of Escherichia coli, and its correlation with the manifestation of diarrhea was analyzed for newborn calves fed these colostrums. The 4 colostrums fed to newborn calves having diarrhea within a few days after birth had significantly lower agglutinin titers for all the serotypes tested than did the other colostrums. Other newborn calves fed on the remaining 12 colostrums did not manifest clinical signs of enteric disorders. The agglutinin spectrum against the E coli serotypes was similar between colostrum samples from the same stockfarm, but variable among the farms from which the colostrum had been collected. The agglutinin activity was sensitive to 2-mercaptoethanol and was found mainly in the macroglobulin fraction of gel filtration, indicating that the activity was due to antibodies of the immunoglobulin M type. This was evidenced directly by the agglutination test of purified immunoglobulins. Opsonic activity of colostrum and immunoglobulins purified therefrom was estimated by the rate of decrease in the number of viable E coli injected into mouse peritoneal cavity. The results indicated that the opsonic activity in colostrum was also attributed mainly to immunoglobulin M antibodies, although the contribution of immunoglobulin A and G antibodies was not ruled out. A part of the opsonic activity in colostrum seemed due to heat-labile component(s) as well.