By means of an automated anti-C3c consumption technic, we quantitated the molecules of the third component of human complement (containing the C3c fragment) that accumulate on red cells (RBCs) during liquid whole-blood storage. Calibrated C-3 sensitized zymosan particles ( ZyC3 ) were used as standards for bound C3. Although the dose-response curves of anti-C3c neutralization by ZyC3 and stored RBCs were similar, mathematical analysis showed that the shapes of these two sigmoidal curves were significantly different. This indicated that the large C3 molecules bound to stored RBCs differed antigenically from those bound to ZyC3 . Despite this difference, the former could be quantitated adequately by the automated anti-C3c consumption technic. Red cell-bound large C3 molecules were measured following various periods of storage at 4 degrees C of whole blood samples (n=102). An average of 48 molecules were detected on RBCs stored for 21 days. Statistical analysis of these data indicated that during storage at 4 degrees C there was a continuous accumulation of C3 on RBCs, followed by cleavage of C3c fragments. The degree of agglutination of stored RBCs with anti-C3c was proportional to the number of cell-bound large C3 molecules.