Decay-accelerating factor (DAF) is a 70,000 MW membrane protein that regulates the complement system on the cell surface. In the present study, we found that DAF had no effect on the classical pathway C3 and C5 convertases that had been stabilized by C4 nephritic factor (C4NeF). In DAF-incorporated cells, however, the assembly of the classical pathway C3 convertase was markedly inhibited even in the presence of C4NeF. C3 nephritic factor (C3NeF) in the alternative pathway protected the C3 convertase from the action of DAF to some extent, while the generation of C3 convertase was also inhibited by DAF. These results indicate that under physiological conditions, DAF functions to inhibit the assembly of C3 convertases even in the presence of nephritic factors, although it has no or little effect on the stabilized convertases. Thus, it is likely that DAF plays an important role in protection of host cells from damage by autologous complement in patients with nephritic factors.