Relative gelatin, collagen type I and collagen type IV degrading activity in small intestinal tissue. Duodenal (A–C), jejunal (D–F) or ileal (G–I) intestinal tissue lysates from either control birds or resin acids fed birds were incubated for 10 h with fluorescently labelled gelatin (A, D and G), collagen type I (B, E and h) or collagen type IV (C, F and I). Breakdown of the fluorescently labelled substrate results in an increase of fluorescence which is proportional to the substrate degrading activity of the sample. Data represent the mean (± standard deviation). FU: relative fluorescence units after 10 h incubation. *p < 0.05, **p < 0.01.