Four different heparin assay methods were compared. Only procedures which can be routinely carried out in clinical laboratories were taken into consideration. A range of heparin concentrations was chosen for these tests which is likely to be expected in low dose heparin prophylaxis. The lowest sensitivity was found when the thrombin clotting time was applied, and slightly better results were obtained by the aPTT method. Addition of purified antithrombin III to the reaction mixtures improved the sensitivity of both methods. When factor Xa was used in a clotting test, the results were comparable to the aPTT method. The sensitivity was however, about 10 times higher when the remaining factor Xa was directly measured in a photometric assay system with a chromogenic substrate. The advantages and disadvantages of the different methods and the usefulness of heparin assays for clinical purposes are discussed.