We have measured the protective effect of colostrum of vaccinated cows with the K99 antigen comparatively with the colostrum of non vaccinated cows, in calves receiving at birth 2 liters of colostrum and just after 5 times 10(10) to 1 times 10(11) E. coli Ent+, K99+. The K99 antigen was prepared from E coli B41 (0101:K99:H-) cultured in a fermentor. Cows were vaccinated subcutaneously with 100 mg or 300 mg (wet weight) in emulsion in Freund uncomplete adjuvant 45 and 15 days before parturition. Of 4 calves receiving a colostrum of non vaccinated cows 4 had a diarrhoea, 3 became dehydrated and 2 died. Of 6 calves receiving colostrum of vaccinated cows, 5 were healthy and the 6th calf had a mild diarrhoea for few hours and became again healthy. Control and protected calves excreted E. coli K99+, 24 HOURS AFTER INFECTION. The protection is probably due to K99 antibodies which inhibit adhesion of E. coli K99+ to the intestinal epithelium.