The beneficial effects of forages containing condensed tannins (CTs) on ruminants are well documented, but the chemical features of CT that yield benefits have not been defined. Some evaluations of limited numbers of highly purified compounds have resulted in positive correlations between CT molecular weight (M W) and biological activity, while others have failed to show a correlation. The objectives of this study were to determine if M W of CT could predict biological activity relative to protein precipitability. M W of condensed tannin, protein-precipitable phenolics (PPP), and the amount of protein bound (PB) were determined for nine species of warm-season perennial legumes. There was no correlation between PPP or PB and M W (R 2 0.11 and R 2 0.02, respectively). However, CT concentration did correlate with PPP and PB (R 2 0.81 and R 2 0.69, respectively). It was concluded that CT M W does not explain the variation in protein precipitation by CT from the forage legumes surveyed.