Abstract.Lesions induced by a bovine coronavirus-like agent were studied in gnotobiotic and colostrum-fed calves using gross, histologic and electron microscopic procedures. Lesions in gnotobiotic calves were present in the colon, mesenteric lymph nodes and in all segments of the small intestine. Calves killed 4 h after the onset of diarrhea had immunofluorescent epithelial cells on the villi of the small intestine and surface of the colon. Calves killed at 44 h had shortened intestinal villi and cuboidal epithelial cells. The villusto-crypt ratio in the lower small intestine averaged 1 .O compared with 5.3 in a control calf. Immunofluorescent cells were present at the tips of the villi, and at the surface and in the crypts of the colon. Colostrum-fed calves that had serum-neutralizing antibody titers for the coronavirus-like agent developed diarrhea when inoculated orally with the agent. There was good correlation between histologic, immunofluorescent and electron microscopic iindings.Diarrhea occurred in calves 5-21 days old on several ranches during a 1971 field trial of an oral neonatal calf-diarrhea vaccine prepared from a reovirus-like agent [9]. Fecal smears from diarrheic calves were immunofluorescent negative for this agent. Diarrheic fecal material from one of the problem herds inoculated into the duodenum of a hysterotomy-derived colostrum-deprived calf caused diarrhea. Feces from this calf contained coronavirus-like particles. Subsequently diarrhea was produced in eight additional calves [9, 111 with fecal material from a calf with experimentally induced diarrhea. Pathological changes in calves inoculated with the coronavirus-like agent are described.

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