Oral administration of streptomycin is known to enhance the susceptibility of mice to enteric pathogens by altering the indigenous flora. We examined the effect of oral streptomycin treatment on the susceptibility of inbred C3H/HeN mice to infection with Serpulina (Treponema) hyodysenteriae. A total of 56 mice were randomly divided into four groups (A-D) of 14 each. From days 0 to 7, mice in groups A and B received streptomycin in their drinking water and mice in groups C and D served as controls. On day 7, mice in groups A and C were inoculated intragastrically with S. hyodysenteriae serotype 4, strain A1, and groups B and D served as uninoculated controls and received sterile trypticase soy broth. Clinical signs were monitored daily and body weights were recorded weekly. Mice were euthanized and necropsied for bacteriologic and histopathologic examinations on day 7 (2/group) and on days 14, 21, 28, and 35 (3/group) of the experiment. Soft fecal pellets were noticed in infected groups (A and C), but no significant differences in body weights were observed between groups (P greater than 0.05). Macroscopic changes were noted only in infected groups (A and C) beginning on day 21 of the experiment and consisted of catarrhal typhlitis, cecal emptiness, and atrophy. Histologically, the cecum and colon of mice in groups A and C had goblet cell hyperplasia, which preceded crypt epithelial cell hyperplasia, inflammatory cell infiltrates, and focal necrosis of mucosal epithelium. S. hyodysenteriae was reisolated from 10 of 12 mice in each infected group (A and C) from day 14 (7th day postinoculation) through day 35 (28th day postinoculation).(ABSTRACT TRUNCATED AT 250 WORDS)