We have used direct binding isotherm analyses to measure the association constant (K(a)) and number of binding sites for the binding of prepared complement-fixing antiobody (Ab)/dsDNA immune complexes (IC) to human red blood cells (RBC). In order to generalize this study we have examined the binding reaction for a number of different anti-dsDNA Ab (from systemic lupus erythematosus plasmas), complement sources, RBC donors, and dsDNA sizes. The affinity of the IC for the RBC is quite high, and the K(a) values fall within a narrow range (5 to 14 x 1010 liter/mol). Similarly, the limiting stoichiometries for the number of IC bound per RBC were between 40 and 91. The very high affinity and limiting stoichiometries both suggest that the IC bind to the RBC via multiple contacts with clusters of complement receptor type 1 (CR1). Furthermore, we have used three specific monoclonal AB (mAb) to quantitate CR1 on human RBC in the presence and absence of bound IC. One of these Ab, mAb 1B4, is blocked from binding to the RBC if IC are previously bound, and we have used this observation to verify the multivalent nature of the interaction of complement-fixing IC with CR1 on human RBC.