The effect of native (n) and heat denatured (d) beta-lactoglobulin, alpha-lactalbumin, and bovine serum albumin (BSA) on activity of bovine plasmin was examined. When assayed on a synthetic peptide substrate, inhibition of plasmin activity by whey proteins was observed at pH 6.5, while at pH 5.2 in the presence of 5% NaCl whey proteins apparently increased plasmin activity. However, urea polyacrylamide gel electrophoresis and RP-HPLC analysis of plasmin digests of P-casein at pH 5.2 indicated that whey proteins inhibited plasmin activity. This inhibitory effect was concentration-dependent, except in the case of nBSA. Thus, the apparent effect of whey proteins on plasmin activity was different when assayed by different methods. Hydrolysis of both nBSA and dBSA by plasmin was observed.