Proteolytic degradation of protein products causes many problems in the bioprocess industry. The increasing production of proteins in heterologous hosts through the use of recombinant DNA technology, has recently brought the problem into focus, since it seems that heterologous proteins are more prone to proteolysis. The result of proteolytic attack may vary from complete hydrolysis of the product to minor truncation of the protein without impairment of its biological function. The degree to which such partial proteolysis is a problem depends on the requirements of the ultimate use of the protein product.