A method of treating patients in whom an inherited disorder results in lack of an essential ENZYME such that an unwanted chemical is deposited in body cells, interfering with their function, by replacing the missing enzyme by intravenous injections. Most of these conditions are classified as LYSOSOMAL STORAGE DISEASES. Currently, such therapy is available for, GAUCHER'S DISEASE types I and III, certain forms of MUCOPOLYSACCHARIDOSIS and one form of glycogen storage disease. Many other types of enzyme replacement therapy are in development. Oral replacement of digestive enzymes are also included under this term, for example, daily ingestion of pancreatic enzymes in CYSTIC FIBROSIS.