Five widely used activated partial thromboplastin time (APTT) reagents (Actin-FS, Actin-FSL, Hemolab Silimat, IL-Test APTT Ellagic Acid and Thrombofax Activated) were compared for their sensitivity and precision in measuring the effect of dermatan sulphate on blood coagulation. On each of 4 days, aliquots of the same normal human plasma pool were mixed with dermatan sulphate (MF701) at concentrations ranging from 10 to 100 micrograms/ml, and APTT was measured in duplicate with all reagents by a photo-optical coagulometer. The order of testing between and within reagents was changed every day. The relationship of APTT ratio to dermatan sulphate concentration was linear with all the reagents. There were statistically significant differences between reagents in their sensitivity to DS, as reflected by linear regression slopes. IL-Test was the most sensitive reagent. At dermatan sulphate concentrations of 20, 50, and 80 micrograms/ml APTT ratio ranged from 1.5 to 1.7, 1.9 to 2.3 and 2.3 to 2.9, respectively, according to the reagent. The lambda coefficient and coefficient of variation derived from regression analysis, both reflecting assay precision, ranged from 0.57 to 0.71 and from 4.6 to 5.1%, respectively, with all but the least precise reagent. The best sensitivity/precision balance was displayed by IL-Test. The APTT reagent should therefore be standardized, with special regard to sensitivity to DS, when testing the relationship of dermatan sulphate clinical effects to APTT response.