Protein determinations on protein-tannin complexes after protein isolation (gel filtration and trichloroacetic acid [TCA] precipitation) or phenolic extraction (polyvinyl pyrrolidone [PVP] and organic solvent precipitation) were unsuccessuful. Kjeldahl determinations of the amount of unprecipitated protein bovine serum albumin [BSA] showed a sigmoid relationship with increasing concentrations of tannins. A similar relationship was found for the reduced viscosity of BSA and plant protein, and the concentration of tannin. Non-linear regression and curve normalization allowed three variable (k 1, k 2 and T 1/2) to be defined for the quantification of the protein-tannin interaction/s. Such a treatment may be useful in studies of the role of tannins in plant-herbivore interactions.