Detection by mAb H206 of C3b-containing complexes formed in normal serum on complement activation in vitro. Complement was activated for the given times in 20% normal serum (NS) or NS supplemented with 10 mg/mL IgG (NS + IgG). Samples were denatured and run along with unreduced C3 (C3, MW 185 kDa) on SDS-PAGE (6% acrylamide) and a blot from this gel was incubated with ¹²⁵I-iodinated mAb H206. An autoradiograph of the blot is shown, revealing the α′C3b-containing protein complexes migrating above α′C3b (MW 112 kDa) as indicated by the bracket, referring to samples run in reduced form. For an explanation of the circled letters, see Figure 1 legend.