Fourteen monoclonal antibodies (MAbs) of putative specificity for human IgG allotypes and isoallotypes were evaluated for reactivity and specificity in 8 different assay systems. The study showed that the MAbs tested could be classified into 1 of 4 groups: those exhibiting allotypic specificity regardless of the assay system, allotypic specificity dependent on the assay system, isoallotypic specificity, and those showing neither allotypic nor isoallotypic specificity. These observations were presumably dependent on antigen presentation, epitope integrity and/or antibody multispecificity. For the G1m(a), G1m(f), G1m(z), G3m(g) and G3m(u) specificities, MAbs have been produced which can be used for routine typing purposes in defined haemagglutination and enzyme-linked immunosorbent assay systems. MAbs are also available that show 'non-g' isoallotypic specificity.