Thirty-two castrated male crossbred growing pigs (average initial wt 26.9 kg) were used to determine the effect of a high level of dietary protein (37%) compared with a normal level of protein (15%) on enterobacteria and Campylobactersp. inhabitation in the large intestine and on visceral organ hypertrophy and the interrelationships between these two factors. Pigs were kept in pairs (eight pens of two pigs/diet) and fed their respective diets ad libitum. Eight pigs (two pens of two pigs fed each diet) were killed at wk 4, 8. 12 and 16 without fasting. Fecal samples were obtained every 2 wk from animals scheduled for nec ropsy at 16 wk, and colon contents were obtained from all pigs at necropsy; samples were enumerated indi vidually for enterobacteria and Campylobacter sp. Weights of heart, lungs, liver, kidneys, perirenal fat and empty stomach, small intestine and large intestine were recorded at necropsy. Stomach, cecum and prox imal colon were sectioned for histopathologic exami nation. Daily body weight gain was depressed by high dietary protein, but liver and kidneys were heavier in the high protein group than in controls at each time interval. Mild lymphoid hyperplasia of Peyer's patches in the small intestine in some pigs in both groups was indicative of antigenic stimulation but not of pathologic significance. There was no effect of diet on counts of enterobacteria or Campylobacter sp. in feces or colon contents during the 16-wk experiment. We conclude that the hypertrophie response of the tissues of grow ing pigs to high dietary protein is not the result of the presence of Campylobacter sp. or enterobacteria in the colon contents.