A study of the serologic activity and molecular structures of three spleen-derived mouse IgA monoclonal human blood group-specific supernatants was undertaken; this was part of an evaluation of these monoclonals as blood typing reagents. The monoclonal antibodies were eluted through a precalibrated size-exclusion column, and fractions were analyzed by immunoblotting, heavy and light chain-specific enzyme-linked immunosorbent assay, and liquid- and solid-phase serologic tests. Results indicated that one of the supernatants (anti-A specificity) contained tetrameric and monomeric forms of IgA, while the other two (anti-A,B specificity) contained three higher polymeric forms (1000-4000 kDa) and one dimeric form. The tetrameric and polymeric forms showed red cell agglutinating activity, whereas the dimeric and monomeric forms did not. All forms contained heavy and light chains. The monomeric anti-A showed specific binding to appropriate red cells in a solid-phase assay, but the dimeric anti-A,B fractions did not. Purified fractions stored at 4 degrees C did not show any equilibration toward other forms, which indicated that the molecules are stable once secreted. The use of such antibodies as blood grouping reagents requires careful monitoring to ensure that high proportions of nonagglutinating molecular weight forms are not produced, as they could compromise the performance of the reagent by binding to red cell antigen in competition with the agglutinating forms.