The production of immunoglobulin A (IgA) protease is a potentially useful marker in differentiating pathogenic from nonpathogenic species of clinical isolates; however, current quantitative assay methods are too tedious for routine application. A simple quantitative method was developed to screen clinical isolates for IgA protease production. This method is based on the specificity of reaction between IgA and alpha chain-specific antiserum in an immunochemistry analyzer (Beckman Instruments, Inc., Brea, Calif.). Colonies of IgA protease producers (Streptococcus sanguis, Streptococcus pneumoniae, Neisseria gonorrhoeae, Neisseria meningitidis, and Haemophilus influenzae) were picked from solid media, transferred to brain heart infusion containing IgA1, and incubated at 37 degrees C for at least 2 h to provide a detectable decrease in IgA concentration. The standard deviation for randomly picked colonies within a species was about +/- 15%. Several IgA protease-negative species caused no detectable reduction in the IgA content of the system. The specificity of the IgA measurement eliminates the requirements for extensive purification and radiolabeling of substrate and provides the basis for a well-defined IgA protease activity unit (micrograms of IgA1 cleaved per minute per milliliter of culture).