An attempt was made to prevent heparin inactivation during the time that lapses between blood collection and laboratory assay. The blood collection in a combination of citric acid, theophylline, adenosine, dipyridamole - C.T.A.D. mixture - was found to reduce greatly the heparin loss during blood centrifugation and storage whatever the temperature for these two steps (centrifugation at 4 degrees C, 12 degrees C or 25 degrees C; storage at 4 degrees C or 20-25 degrees C). With this mixture the influence of centrifugation temperature upon the heparin loss appeared negligible. The heparin loss during the initial 5 hours storage at 20 degrees-25 degrees C was found to be 4.4 +/- 4% for a heparin concentration of 0.45 +/- 0.05IU/ml. The combination of the platelet active drugs with citric acid in C.T.A.D. mixture did not interfere either with the amidolytic heparin assay (anti IIa) or clotting tests such as activated partial thromboplastin time and calcium thrombin time. Thus the C.T.A.D. mixture appears to be useful in routine use because it is effective, cheap, stable, does not interfere with the tests currently used for monitoring heparin therapy and thus helps overcome the main cause of error in heparin assays.