Less than 50 years ago it was discovered that steady-state protein concentrations in plasma are the net result of continuous elimination and synthesis of protein molecules. The first quanti- tative studies on the turnover and distribution of plasma pro- teins were made around 1950, after the introduction of radio- labeled protein preparations. Around 1970, another development in quantitative interpre- tation of circulating proteins was initiated in clinical enzy- mology. Estimation of cumulative release into plasma of cellular enzymes can be helpful in a variety of diseases to assess the...