The number of CR1 on erythrocytes, as measured by monoclonal antibodies, remains undefined because of the repetitive structure of CR1 and the presence of different types of structural CR1 alleles. We studied the number of CR1 per erythrocyte using two monoclonal antibodies, E11 and 3D9, which recognize different sites on CR1. The number of binding sites was higher for E11 than for 3D9 (ratio E11/3D9: 1.9 +/- 0.4, n = 17); however, this ratio was not affected by CR1 numbers or alleles. Partial digestion with papain of CR1 on erythrocytes abolished the binding of 3D9. It reduced the binding of E11 to one-third of its initial value (0.35 +/- 0.03; n = 13) using cells with different CR1 numbers or alleles. By immunoblotting, a unique 75-kDa stump of CR1 remained attached to the erythrocytes for every allele studied. Taken together, these results suggest that the number of CR1 has been over-estimated in the past using E11 by a factor of 2, or more probably 3. However, the over-estimation of CR1 number has been identical for erythrocytes bearing different CR1 numbers or alleles.