Two coagulation tests, Recalcified Activated Clotting Time (RACT) and Hemochron, were compared with the activated partial thromboplastin time (APTT) in in vitro heparinization experiments and in patients heparinized for long term extracorporeal circulation (ECC) to determine their suitability as guidelines for heparin administration. All tests had good precision, with a small variability between methods. There was satisfactory correlation with the heparin level in vivo (r = 0.77 for RACT; r = 0.80 for Hemochron and r = 0.82 for APTT) too, while the in vitro r was always greater than or equal to 0.94. Nevertheless, APTT values for plasma heparin levels higher than 1.5 U/ml were markedly prolonged, with a large standard deviation, and often "unclottable". The good correlation between the three tests, makes it possible to substitute RACT and Hemochron for APTT, during long-term ECC.