mAb H206 recognized α′C3b released by hydroxylamine from C3b-containing complexes. Complement was activated for 3 minutes in either normal serum (NS) or factor I–deficient serum (Fact. I def. S), or in NS supplemented with labeled C3 (¹²⁵I-C3). Denatured sera were subjected to 2-dimensional SDS-PAGE with a hydroxylamine treatment between the dimensions. Blots from 2-dimensional SDS-PAGE were either incubated with ¹²⁵I-iodinated mAb H206 (mAb H206) or stained, when originating from samples supplemented with ¹²⁵I-labeled C3 (¹²⁵I-C3). Autoradiographs are shown. Conditions and arrangement of the 2-dimensional SDS-PAGE are given at the right-hand side. The circled a and b mark the position of the reduced form of known C3b-containing complexes: complex b represents α′C3b₂, originating from dimeric C3b and complex a has earlier been identified as α′C3b₂-HC, originating from a C3b₂-IgG complex¹⁴ ; ai denotes the inactivated form of complex a. The position of α′C3b and 2 of its fragments (86 kDa and 65 kDa) are marked.