Changes in the activity of the complement system components C1--C5, and proteinase inhibitors: alpha 1-inhibitor, alpha 2-macroglobulin and antithrombin III were studied in red blood cell pack during storage at 4 degrees C, in comparison with erythroconcentrate. A decrease, detected in the level of C1, C3 and alpha 2-macroglobulin, was more pronounced in the erythroconcentrate than in the red blood cell pack. The data obtained have evidenced that the adequate amount of plasma plays an important role in the red blood cell preservation and prevention of accumulation of undesirable physiologically active substances in the transfusion solutions.