To test the efficacy of adding enzyme supplements to feeds, an in vitro study was conducted by mixing bovine trypsin or proteinases from the gastric juice of the Cortez swimming crab Callinectes bellicosus with an enzyme extract from the digestive gland of the whiteleg shrimp Penaeus vannamei. Enzymes alone and mixtures were tested for hydrolysing proteinaceous natural substrates (bovine casein, bovine haemoglobin, and bovine serum albumin). All enzyme preparations hydrolysed casein. Shrimp enzymes hydrolysed haemoglobin but not serum albumin. Bovine trypsin and crab proteinases hydrolysed serum albumin but not haemoglobin. The mixture of shrimp and crab enzymes generated more hydrolytic products of serum albumin than shrimp enzymes alone. Shrimp enzymes mixed with bovine trypsin did not hydrolyse albumin because the bovine trypsin vanished; shrimp enzymes hydrolysed bovine trypsin. Results indicated that it is naive to assume that proteinolytic enzymes from different species will add their catalytic capabilities if mixed; here, we demonstrated that they may act as proteinases and will hydrolyse available protein regardless of its function. Our conclusion is that enzyme supplements should be tested in vitro before intending them as exogenous proteinases in feeds. This technique can be used to assess the compatibility and additivity of proteinases when mixed for biotechnological purposes. Besides, the technique can demonstrate who hydrolyses whom.