The authors evaluate the sensibility and validity of some reagents for activated partial thromboplastin time during construction of a curve for measurement of factor VIII coagulant activity. For every reagent three clotting factor VIII deficient plasmas have been used; Baldacci, Dade and Behering. Significant differences in results between every reagent for aPTT and laboratory have been observed. By these results the Authors show the best reagent for aPTT. Future fields of research have been analyzed.