Abstract.Performance characteristics of pooled rabbit IgG polyclonal anti‐C3d are compared with one mouse IgM and three mouse IgG monoclonal anti‐C3d antibodies (MAs). IgG MA,s employed singly or in combination, failed to precipitate C3d; by contrast, IgM MA and polyclonal anti‐C3d precipitated C3d. Measurements of polyclonal anti‐C3d concentration by chemical means and by125I‐C3d radioimmunoassay (RIA) agreed closely. RIA values were 50% of chemical measurement values for three of the four MAs. Use of sucrose density gradient ultracentrifugation to assess MA C3d/anti‐C3d molar combining ratios for soluble anti‐C3d/C3d was not possible because fast‐sedimenting multimeric C3d/anti‐C3d complexes did not form. Dissociation and competitive binding studies indicate that (1) two MAs haad substantially lower affinities than the other anti‐C3d antibodies, and (2) polyclonal anti‐C3d recognizes more C3d epitopes than are recognized by individual MAs. The results demonstrate antigenic complexity of C3d fragment and illustrate the difficulties of predicting individual MA performance based on prior experience with