351 sera from 27 human recipients of renal allografts and 21 healthy blood donors were assayed for circulating immune complexes by the C1q solid-phase radioimmune assay. Increased C1q-binding activity (C1q-BA) was detected in pretransplant sera from 5 patients with chronic pyelonephritis (PN) and 3 patients with chronic glomerulonephritis (GN). A significant decrease of C1q-BA immediately after transplantation could not be found. 6 weeks after transplantation only 2 patients of the PN group showed increased C1q-BA. Serial studies in 17 patients with rejection crises did not show any correlation between the level of serum C1q-BA and the occurrence of rejections. Furthermore, no correlation could be found between the occurrence of complement-dependent lymphocytotoxic antibodies measured by the <sup>51</sup>Cr release technique and the level of serum C1q-BA. In contrast, our results show that the probability of graftectomy or graft failure is significantly higher, at least in the early phase after transplantation, when the serum C1q-BA is lowered for several weeks.