1. Polysaccharide depolymerases and glycoside hydrolases involved in the breakdown of plant structural polysaccharides (hemicellulose and pectins) were monitored in three fractions of the liquid phase of horse caecum digesta: acellular fluid (AF), bacteria (B) and protozoa plus bacteria (PB).
2. Both bacteria and protozoa were found to be involved in the decomposition of pectic substances, with two enzymic activities: depolymerase (polygalacturonase, EC 3.2.1.15; and pectin lyase, EC 4.2.2.10) and esterase (pectinesterase, EC 3.1.1.11). The activity of the PB fraction was higher than that of B.
3. With hemicellulosic substrates, all three fractions showed a significant xylan endo-1,3-β-xylosidase (EC 3.2.1.32) activity. Mannan was hardly broken down.
4. Galactomannan and arabinogalactan were broken down more extensively by the PB fraction than by the B fraction. Glycosidase activities (xylan 1,4-β-xylosidase, EC 3.2.1.37 and α-L-arabinofuranosidase, EC 3.2.1.55) were also observed.