IgA protease produced by various strains of Haemophilus influenzae can digest serum IgA and yield its fragments which can react with anti-IgA serum. We assayed IgA protease activity by detecting the digests of IgA by SDS-PAGE and immunoblotting. The digests were separated with SDS-PAGE, transferred to nitrocellulose membranes and detected with anti- (alpha chain of human IgA, its Fab and its Fc) immunoglobulin conjugated peroxidases. Using this method, we can determine which type of IgA protease is produced by various of H. influenzae strains. All the 20 strains isolated from respiratory tracts produced IgA protease.