Colloid osmotic pressure (COP) of some of the most frequently used plasma replacement fluids was measured with a colloid osmometer. COP of 4% human albumin solutions was only half that of normal human serum (13.6±0.6 vs. 27.5 ± 2.7 mmHg (1.8±0.1 vs. 3.7±0.4 kPa)) (mean±s.d.), whereas COP of 20% human albumin solutions was eight times higher (196.0±12.3 mmHg (26.1 ± 1.6 kPa)). Enhancing the protein concentration from 4% to 20% in the human albumin solutions increased COP 14-fold, reflecting the exponential relationship between protein concentration and COP of a solution. Fresh donor plasma furnished by the hospital blood-bank had a COP about 30% below normal human serum (18.1 ± 1.3 mmHg (2.4 ± 0.2 kPa)), due to dilution during preparation. Dextran 70 (6%) had a COP more than twice, and RingerDextran 60 (3%) about 75% of that of normal human serum. Dextran 40 (10%) and gelatin (3.5%, Haemaccel) leaked markedly through the membrane of the colloid osmometer, making acceptable measurements impossible. Seven different hydroxyethyl starch (HES) solutions were measured, and the COP varied between half and 3 times that of normal human serum, depending on molecular weight and concentration of the HES.