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Various human secretions (intestinal secretion, saliva, nasal mucus, lacrimal fluid) have been found to inhibit the binding of antibodies to their antigens. Various characteristics (e.g. time, pH, temperature dependence, affinity and size exclusion chromatography) suggested that the inhibitory activity was attributable to an enzyme. Further investigations revealed that this enzyme reacted with the Fab portion of immunoglobulin G, specifically with the heavy chain. It is assumed that it represents a novel immunoglobulin-specific protease since similar results were not obtained with proteolytic enzymes from human digestive organs e.g. pepsin, trypsin and chymotrypsin. Finally, investigating saliva it was demonstrated that the putative protease was not identical to enzymes from periodontal bacteria which are proteolytic for the Fc portion of immunoglobulins. The findings could be of general importance in the design of immunoassays which are to be applied to human (and possibly animal) secretions.