We examined the effect of excess factor D on the alternative pathway of complement (APC). First, we demonstrated that the production of C3a is accelerated in the fluid-phase with the addition of purified factor D. Analysis by sodium dodecylsulfate polyacrylamide gel electrophoresis under reducing conditions showed that the serum iC3b level was elevated when incubated with excess factor D. Secondly, we demonstrated, by measuring the C5a-des-Arg level, that the generation of C5a was promoted in the fluid-phase with the addition of purified factor D. We then studied whether activation of APC is elevated in the blood of patients on maintenance hemodialysis whose sera contained a high concentration of factor D. First, we detected, by fluorescence activated cell sorter analysis, greater amounts of C3d on erythrocytes from the patients (mean fluorescence intensity ± SD: 7.7 ± 1.7 arbitrary units) than those from healthy individuals (5.4 ± 0.5 arbitrary units; p < 0.001). Secondly, serum C3 level was significantly lower (p < 0.001) in patients (mean ± SD: 63.3 ± 8.2 mg/dl) than in healthy individuals (84.8 ± 9.5 mg/dl), whereas there was no difference in serum C4 level between patients (32.4 ± 6.9 mg/dl) and healthy individuals (33.0 ± 7.4 mg/dl). Serum C5 level was almost the same in patients (10.5 ± 1.5 mg/dl) and in healthy individuals (11.2 ± 1.3 mg/dl). These results provide supportive evidence of elevated APC activation in patients with high serum