In radioantigen-binding assays for antibody to Haemophilus influenzae type b capsular polysaccharide, an apparent antibody concentration [( (["Ab"]) is assigned to test sera on the basis of binding equivalence to dilutions of standard adult antiserum. In childrens' sera, ["Ab"] can disagree between laboratories and between intralaboratory assays using different preparations of radioantigen. Immunochemical variables causing disagreement include the dilution of test serum at which ["Ab"] is assigned and the concentration and size distribution of radioantigen. Binding-dilution curves of many childrens' sera displayed shallower slopes than did the standard, thus ["Ab"] increased with dilution, particularly in low-binding sera. An increase in radioantigen concentration also raised ["Ab"]. Using larger-sized radioantigen raised the binding in standard and childrens' sera comparably and thus would not directly affect ["Ab"] but would raise ["Ab"] indirectly by increasing the serum dilution. These results are theoretically predicted if the affinity of childrens' antibodies are lower than that of the standard.