The osmolality of plasma and heparinised whole blood samples collected from hospital patients was estimated using measurement of the depression of freezing point. There was no clinically significant difference between osmolality measurement made on either whole blood, or plasma taken from the same patient. Neither cell volume nor haemolysis was found to affect the measurement. The reproducibility of whole blood measurements was similar to that for determinations carried out on plasma. Measurement of osmolality on whole blood is quicker and cheaper and needs a smaller specimen than if serum or plasma is used.