(Table). Thus, it has been reported that heparin activity is constant for 24 hours regardless of the diluting fluid or its pH over a wide range,'-4 or that heparin activity is reduced by 40-50% within 1 hour of dilution except in 0-9 00 sodium chloride,5 or that heparin activity is reduced by 50-60% after 4-6 hours in all diluents but then increases again with further storage.6Because of these contradictory and sometimes confusing reports about heparin stability in vitro we decided to re-investigate the question.Further, because there is disagreement about the most appropriate test of heparin effect, we decided to measure heparin stability in vitro using four tests: the Received for publication 17 December 1979 activated partial thromboplastin time (APTT), the thrombin clotting time (TCT), factor Xa inhibitory activity (Xa time), and protamine sulphate neutralisation. Methods STUDY DESIGNSodium heparin was added to 500 ml 0 9 %0 saline or 5 0 dextrose, supplied commercially for intravenous use in plastic bags or glass bottles, to give a heparin activity of 10, 20, or 40 IU/ml, and to 50 ml 09 %0 saline or 5 % dextrose drawn into a plastic syringe to give a heparin activity of 400 or 1000 IU/ml. These conditions were chosen to mimic those obtaining during heparin infusion by intravenous drip or syringe pump. Heparin activity was assayed immediately and after 3, 6, and 24 hours of room temperature storage. In addition, heparin stability was measured during storage at varying pH.