Differences in rheumatoid factor (RF) activity among the molecular species of IgA were investigated with the use of monomeric and polymeric monoclonal IgA RF paraprotein from the serum of a patient (PS) with idiopathic hyperviscosity syndrome. After fractionation by gel chromatography in acidic buffer, RF activity as determined by latex fixation and solid-phase radioimmunoassay (RIA) specific for IgA RF was confined to be high m.w. (> 7S) fractions. However, after adsorption into polystyrene wells, fractions containing monomeric (7S) IgA, as well as those containing polymeric IgA, bound 125I-labeled heat-aggregated human IgG. These observations were confirmed after further purification of the IgA fractions by passage through a protein A-Sepharose CL-4B column followed by precipitation of the IgA proteins with ammonium sulfate at 50% saturation. After 'cross-linkage' by a hybridoma anti-human α-chain antibody, the activity of the monomeric IgA preparation in the IgA RF RIA approached that of the polymeric IgA preparation. Gel filtration studies verified that this activity was not due to contamination by polymeric IgA RF. Further, classic RF specificity was confirmed for both the monomeric and polymeric IgA RF by reaction with human Fc-coated but not Fab-coated wells. A control monomeric IgA myeloma protein and normal serum IgA did not react in the RF RIA when analyzed in the presence or absence of the hybridoma anti-α-chain antibody. Moreover, the activity of the polymeric IgA RF preparation from patient PS in the RIA was minimally influenced by the hybridoma antibody. These results indicate that IgA RF can coexist in both polymeric and monomeric forms, demonstrate that monomeric IgA RF may escape detection by previously described RIA techniques, and suggest an approach for its detection.