SummaryThe small intestine of 51 calves was examined for the presence of K99+Escherichia coliby means of both an immunoperoxidase procedure performed on paraffin sections and by the slide agglutination test after isolation. Twelve cases resulted immunoperoxidase positive (23.5 %) and 8 of them were also agglutination positive. Results of the 2 diagnostic tests agreed in 46 cases (90.2 %). In immunoperoxidase positive sections a thick layer of immunoreactive bacteria was seen on the luminal surface of the enterocytes.Post‐mortemautolysis or prolonged fixation did not seem to affect the immunoperoxidase reactivity of the sampl