This monograph reviews some 589 papers published mostly during the decade from 1951 to 1961 inclusive with regard to the possible clinical significance of plasma enzymes. It appears that plasma enzymes are either the result of cell secretion such as acid and alkaline phosphatase, amylase, pepsin, and a few others or they are lost from cells undergoing sudden disintegration or reversible increase in permeability. The recent decade has added little to our knowledge and ability to use the secreted enzymes. The bulk of recent literature deals with plasma enzymes which leak out of cells and probably have no specific purpose in plasma. Much search has been made for organ specific and disease specific plasma enzymes or combinations of enzymes. In general the plasma enzyme levels are of greatest use in situations where the site of enzyme loss is either established by other diagnostic information or at least highly suspect. It