A fat-free diet (FFD) reduces bacterial flora when compared to a regular diet (RD) in rats. FFD reduces the degree and incidence of indomethacin-induced intestinal ulceration concomitantly with a reduction in bacterial flora overgrowth produced by the drug. Present and previous findings indicate that diminished deconjugation of indomethacin and reduction of intestinal flora overgrowth play a major role in the ulcer-preventing properties of FFD.