The effect of native (n) and heat denatured (d) whey proteins on chymosin activity was investigated using both a synthetic heptapeptide and alpha(S1)-casein as substrates. At pH 5.2, in the presence of 5% NaCl, both native and denatured beta-lactoglobulin (beta-lg) and bovine serum albumin (BSA) inhibited chymosin activity against the synthetic substrate, while apparent stimulation of chymosin activity was observed in the presence of all whey proteins at pH 6.5, and in the presence of native and denatured a-lactalbumin (alpha-la) at pH 5.2. Analysis of chymosin digests of alpha(S1)-casein by urea-polyacrylamide gel electrophoresis and reverse phase-HPLC indicated inhibition of chymosin activity at both pH values by native or denatured whey proteins. Greatest inhibition was observed by the addition of native or denatured BSA at pH 6.5. Thus, the effect of whey proteins on chymosin activity, when assayed by different methods, gave different results. Hydrolysis, of both nbeta-lg and dbeta-lg by chymosin was observed at pH 6.5.