The biological effect of tannins (proportion of the response in different parameters when tannins were inactivated with polyethylene glycol, PEG) as an easy, rapid way to estimate the magnitude of their effect on rumen microbial fermentation, was estimated in vitro for the tropical browse legumes Albizia lebbekoides, Acacia cornigera and Leucaena leucocephala, which differ in their phenolic and tannin content. Samples were incubated in rumen fluid for 24 h in four runs. The inactivation by PEG of tannins from A. lebbekoides increased gas production from 1.62- to 2.83-fold, with this biological effect increasing up to 8 h incubation, then being maintained and increasing after 16 h. In A. cornigera and L. leucocephala, the magnitude of the improvement of gas production was lower (from 1.1- to 1.32-fold and from 1.29- to 1.56-fold) and constant. The inclusion of PEG increased total volatile fatty acids (VFA) concentration (P = 0.019), reduced the molar proportion of acetate (P < 0.001) and increased that of butyrate (P < 0.001) and branched-chain VFA (P < 0.001). Microbial protein mass in A. lebbekoides increased with PEG in a higher extent (P < 0.001) than in L. leucocephala, but it was reduced in A. cornigera. No biological effect was observed on the efficiency of microbial protein synthesis when it was related to VFA concentration (P > 0.10), but when related to the gas produced it was lowest with A. lebbekoides (P = 0.023). The biological effect of tannins, either in total extent or along the incubation period differed according to their origin. Irrespective of their amount or chemical nature, the biological effect gives a direct idea of how tannins affect both the extent and pattern of forages fermentation.