Specific-pathogen-free pigs were examined for susceptibility to swine dysentery after daily treatment with aflatoxin B1 (0.07 to 0.14 mg/kg). In the pigs (group II) given both aflatoxin and Treponema hyodysenteriae, the incubation period before the onset of swine dysentery was shorter than that in pigs (group III) given T hyodysenteriae alone. Also, the number of days in which infected pigs showed signs of diarrhea and dysentery was greater in the group given aflatoxin and T hyodysenteriae than in the group given T hyodysenteriae alone. Four of 8 pigs given aflatoxin and T hyodysenteriae died during the experiment, whereas only 1 of 8 infected pigs in the other group died. Pigs in both groups, convalescent from swine dysentery, were immune to rechallenge exposure. Subclinical effects of aflatoxin consumption were evident at necropsy, but clinical signs and lesions of swine dysentery were not observed in group III pigs (given aflatoxin only).