We have demonstrated previously that during 4 degrees C storage of blood, RBC becomes sensitized with the third component of human complement containing the C3c fragment. In studies done during the first year we have demonstrated that there is a progressive increase in RBC-bound C3 that is insensitive to the action by C3b inactivator. A series of autologous 51Cr survival studies (n equals 31) has revealed a statistically significant correlation between 24-hour survival and the quantity of C3b inactivator-resistant C3 on RBC. These data strengthen our hypothesis that the removal of RBC damaged during storage is mediated by cell-bound C3b. We plan to explore this association further by means of both 51Cr and Ashby RBC survival studies, and by recovering the donor RBC after transfusion for purposes of testing. The relationship of the quality and the quantity of cell-bound C3 to RBC survival will be tested. In order to clarify the mechanism of complement deposition to RBC during 4 degrees C storage, experiments will be performed to explore the pathway that leads to activation of C3. Effects of storage temperature and blocking of either the alternative or classical pathways on the above-described phenomenon will be explored.