Anti nutritional factors in tropical legumes are very common and the chemical analysis for tannins has become an important tool to evaluate alternative ruminant feeds in the tropics. However, frequently results from tannin chemical analysis are not in agreement with biological response when animals are fed those feeds. There is a lack of information concerning the biological effects (activity or reactivity) of tannins on ruminants. Usually the effects of tannins are tested in vitro by adding compounds with capacity of biding tannins. The aim of this work was to compare two binding compounds to evaluate the effect of tannins of tanniniferous feeds for ruminants.