The C3b/C4b receptor, also known as complement receptor type 1 (CR1, CD35), is a single chain glycoprotein consisting of 30 repeating homologous protein domains known as short consensus repeats (SCR) followed by transmembrane and cytoplasmic domains. A series of recombinant proteins derived from CR1 has been prepared and assessed for the capacity to inhibit complement lysis of the host Chinese hamster ovary (CHO) cells. The full-length recombinant CR1 inhibited human complement-mediated CHO cell lysis, and the efficiency of inhibition was directly proportional to the number of receptors/cell. The SCR 15-18 of CR1, but not SCR 15-16, inhibited complement lysis of the host CHO cell, bound monomeric C3b (Kd,app = 6.5 x 10(-7) M), and dimeric C3b (Kd = 1.8 x 10(-8) M), and served as a cofactor in the proteolysis of C3b by factor I, confirming and extending the observations of Fearon and colleagues (Kalli, K. R., Hsu, P., Bartow, T. J., Ahearn, J. M., Matsumoto, A. K., Klickstein, L. B., and Fearon, D. T. (1991) J. Exp. Med. 174, 1451-1460). The SCR 1-4 of CR1, but not SCR 1-2, also inhibited complement lysis of the host CHO cell, indicating that more than two SCR are necessary and that four SCR are sufficient for optimal C4b binding to CR1. Thus, the structural requirements for C4b binding are analogous to those for C3b binding, namely, four SCR of CR1 form the binding sites for each of these proteins. CR1 has long been recognized to regulate extrinsic complement activation, that is, to bind to and promote the degradation of fluid phase C3b and of C3b attached to immune complex. These results demonstrate that CR1 is also an intrinsic regulator of complement activation in that, under appropriate conditions, CR1 inhibits complement-mediated lysis of the cell on which it is expressed.