This work studies the quality of the response of the Electra 600 for the determination of the prothrombin time (PT) and the activated cephalin time (ACT). The intra-serial precision was good for the PT and the ACT with normal or pathological plasmas (CV 2%). The precision was excellent for the PT (r = 0.98; Electra 600-fibrometer). The correlation coefficient varies between 0.85 and 0.95 for the ACT depending on the nature of the activator chosen. Lactescent plasmas having a protein concentration greater than 85 g/l should be treated manually for measurement of the PT. No interference was noted with other biological substances: eg glucose, bilirubin, hemoglobin. The increasing addition of heparin indicates a correct sensitivity of the response with Electra 600 both for short and long ACT. Autoanalysis renders the determinations of PT and ACT independant of the manipulator, that of the PT being much more rapid than with the manual method