The complete coagulation profile of citrate phosphate dextrose (CPD) stored blood has been studied during 21 days storage at 4°C. Ten donations were studied at zero, one, seven, 14 and 21 days. One donation contained small clots and was studied separately. Activities of factors I, II, IX, XII or XIII did not deteriorate significantly. Slight, but clinically unimportant deterioration occurred in factors VII, X and XI. There was marked deterioration in activities of factor V, and factor VIII, the levels decreasing from 1.0μ/ml at day zero to 0.51 to 0.30 respectively at day 21. The donation which contained small clots had more marked decreases in factors V and VIII from the first day. The general stability of coagulation factors suggests that even massive blood replacement is unlikely to result in clinically significant coagulation defects.