A modification of a linear diffusion test for measuring milk clotting enzymes at concentrations of 1 x 10^-4 to 1 x 10^-1 chymosin units/ ml was developed to permit quantitative assay of porcine pepsin in Cheddar cheese with a standard deviation of 6%. The amount of porcine pepsin retained in Cheddar cheese curd was dependent on pH of milk at setting. Milk at pH 6.6, 6.4, 6.2, and 6.0 was set with porcine pepsin, acidified with lactic acid and glucono-~-lactone and made into Cheddar cheese. After pressing the corresponding curd contained 0, 3.64+0 .12 %, 4.79+0.52%, and 5.94+0.30% of the pepsin activity added to the milk. Polyacrylamide gel electrophoresis of cheese revealed increasing degradation of the ~s-casein band with increasing residual pepsin in the curd. However, some degradation of the ~s-casein band was evident in curd set at pH 6.6 which showed no residual pepsin. Curd was made by ultrafiltration from whole milk followed by acidification to pH 5.2 with hydrochloric acid and glucono-6- lactone and vacuum evaporated to 39% moisture at 40 C (no clotting enzymes and no starter) . Degradation of the ~s- casein in this curd was similar to that observed in pepsin cheese set at pH 6.6. All degradation in ~s-casein cannot be attributed to milk clotting enzymes or starter bacteria. Porcine pepsin does not contribute to protein digestion in cheese curd during ripening unless the milk is below pH 6.6 at setting.