A method for routine determination of colloid osmotic pressure (COP) in the clinical laboratory is described. A transducer membrane system is utilized in which the colloid osmotic pressure (COP) of fresh or refrigerated plasma or albumin is compared to that of isotonic saline. The measurement is completed within an average period of less than four minutes. Duplicate measurements demonstrated high reproducibility (r = >.95) and reference measurements obtained over a period of ten months were consistently within a range of 1 torr. Heparin in excess of 200 units/ml delayed equilibration. Values obtained in normal ambulatory and supine subjects conform closely to those previously reported.