Decay-accelerating factor (DAF) is a membrane glycoprotein found on various cells that are in contact with complement. It inhibits the formation of the C3 convertases of the complement system, both the classic (C4b2a) and alternative (C3bBb) pathways. In this investigation, we used a homobifunctional cross-linking reagent to search for a DAF ligand on the surface of cells subjected to complement attack. We found that DAF forms complexes with C4b and C3b deposited on the same erythrocytes, but not with the physiologic degradation products of these complement fragments, that is, C4d or C3dg. Taken together with prior observations that DAF action is reversible, and DAF does not affect the structure of C4b or C3b, these findings suggest that DAF functions by competitively inhibiting the uptake of C2 or factor B, and preventing the assembly of the C3 convertases.