Abstract: This study compared 38 sainfoin and 2 Lotus accessions to their respective tannin contents, N buffer solubility, and in vitro protein degradation. Tannin contents were measured by a protein precipitation method using either bovine serum albumin or Rubisco and by the colorimetric HCl/butanol method. Precipitation of bovine serum albumin and Rubisco was highly correlated (R² = 0.939). Correlations between the protein precipitation variants and the HCl/butanol method were relatively low (R² < 0.6). Protein degradation was measured at 4 h of incubation in an inhibited in vitro system and could not be explained by any of the tannin assays (R² < 0.03) and only partially by N buffer solubility (R² ≤ 0.433). Decisive factors other than the quantity of tannins or their ability to precipitate proteins must be considered. Resistance of soluble protein toward degradation can possibly be caused by tannin protein binding.