Pure cultures of Campylobacter fetus subspecies jejuni of bovine origin were used to inoculate three ruminating calves in each of two experiments and three milk-fed calves in another. Inoculated animals all developed clinical signs which included fewer (to 41 degrees C), diarrhoea and sporadic dysentery within one to three days of inoculation. Diarrhoeic faeces were dark, mucoid, of uniform consistency and were passed for six to 15 days. The eight control animals did not develop this syndrome. C f subspecies jejuni was isolated from the faeces of all nine inoculated animals and from the faeces of two control animals in one experiment. The animals were killed 10 to 16 days following inoculation and found to have thickening of the wall of the ileum, varying degrees of inflammation of the jejunal and ileal mucosa, dark, mucoid ileal contents and enlargement of the mesenteric lymph nodes. Stunted villi, dilated crypts, some of which were filled with inflammatory cells, dilated capillaries and mononuclear cell infiltration were seen in affected small intestinal mucosa. C f subsp jejuni was isolated from the ileum, caecum and colon of all the inoculated animals and less frequently from the jejunum, gall bladder and abomasum. Antibody to the inocular strain of C f subsp jejuni was demonstrated at titres of at least 1:640 in the serum of all inoculated animals and was absent from all the control sera. The probability that C f subsp jejuni caused the syndrome produced was discussed.