The blood group substances Chido (Cha) and Rogers (Rga) represent two electrophoretic variants of human C4. Based on the observation that anti-Cha and anti-Rga antisera agglutinated human red blood cells prepared in sucrose-activated autologous serum (LIS cells) at 37 degrees C, it has been assumed that the Cha and Rga antigenic determinants reside in the C4d fragment of C4. Here, we present evidence indicating that C4d is not present on those cells. In order to identify structurally the C4 fragments deposited, LIS cells were prepared at 37 degrees C and 4 degrees C in autologous serum to which 125I-C4 was added. Membranes of LIS cells were solubilized and analyzed by SDS-PAGE in 5 to 15% gradient gels followed by autoradiography. C4d was not deposited on LIS cells prepared at 37 degrees C, whereas C4c (beta, gamma, alpha 3 alpha 4) was. Cells prepared at 4 degrees C carried C4d (alpha 2) and C4c. Anti-Cha and anti-Rga antisera agglutinated both cell types, although C4d was not present on the cells prepared at 37 degrees C. Purified C4, C4c, C4d, and alpha-, beta- and gamma-chains of C4, as well as alpha 3 and alpha 4, were used to neutralize these antisera. C4 and the alpha-chain C4d and alpha 4 fragment of C4c, but neither the alpha 3 fragment nor the beta- or gamma-chains, were capable of neutralizing anti-Cha and anti-Rga antisera. These results strongly suggest that C4d and alpha 4 share an antigenic determinant, both of which are recognized by anti-Cha and anti-Rga antisera.