We studied complement functions, breakdown of C3 and aggregation of immunoglobulins in whole blood stored in citrate phosphate dextrose at 4 degrees C for up to 3 weeks from 10 normal blood donors. No significant changes in total haemolytic complement (CH50) and alternative pathway haemolytic complement activity were detected. However, the complement-mediated capacity of the sera to solubilise a pre-formed immune precipitate decreased significantly at 2 and 3 weeks (p less than 0.02 at 2 weeks, p less than 0.003 at 3 weeks). No C3 conversion could be detected by immunoelectrophoresis but alteration in C3 was attested by a significant increase in breakdown products of C3 by 1 week (p less than 0.002) and all were above the normal range after 3 weeks (p less than 0.0001). No aggregation of immunoglobulins could be detected using two immune complex assays.