A subtype specific ELISA using purified hemagglutinin (HA) from influenza A H1N1 and H3N2 was developed to detect antibodies present in swine previously exposed to either H1N1 or H3N2 influenza viruses. The HA was extracted using the detergent octylglucoside followed by ion exchange chromatography. All HA preparations were free of contaminating nucleoprotein and matrix protein contamination. Monospecific swine anti-H1N1 and swine anti-H3N2 sera were used to demonstrate the subtype specificity of the assay. Monospecific rabbit anti-H1N1 or H3N2 was used to sterically block crossreacting determinants and thus enhance assay specificity. A linear relationship between single dilution point ELISA and the hemagglutination inhibition (HI) test was established. This enabled the accurate estimation of HI titer from ELISA. Further refinement of this ELISA based HI estimation system could allow it to replace the current HI procedures in instances where identification at the subtype level of specificity is acceptable. The substantial specificity requirements associated with the detection of strain specific antibody would still necessitate the use of the HI procedure.