Crude cell extracts contain a number of endogenous enzymes, such as proteases and phosphatases, which are capable of modifying the proteins present in the extract. The best way to improve the yield of native proteins is to add inhibitors of these enzymes known to be present in the source material. This phosphatase inhibitor cocktail III has been optimized and tested for serine and threonine phosphatases, acid and alkaline phosphatase.