AbstractSolid‐phase immunoassay‐derived antibody titers are often converted to weight unit concentrations with the aid of standard sera containing known antibody concentrations. Systematic studies justifying this procedure have not yet been published. We therefore investigated the magnitude of errors associated with this conversion. Antibody concentrations of thirteen sera or ascites fluids were determined by quantitative precipitation or equlibrium dialysis, and one was then used as a “standard antibody” for the others in solid‐phase radioimmunoassay (SP‐RIA) or enzyme‐linked immunosorbent (ELISA) assays.Antibody concentrations determined by the conventional solid‐phase assay (the “standard serum” has the same specificity as the “sample”) had up to fourfold errors. These errors could be reduced by basing the conversion on the combination of two standard sera instead of one.The possibility was studied of whether the conversion to weight units could be done with the aid of a standard serum directed to a different antigen than the sample antibody. Errors associated with the use of such a heterologous standard were not significantly greater than those found using the conventional conversion. A combination of two reference sera again reduced the errors. The use of such heterologous standard(s), however, requires checking the binding capaci