An immunoradiometric assay using two monoclonal antibodies directed to human trypsin 1 was developed for measuring trypsin(ogen) in biological fluids. The assay is different from other assays in that it is specific for cationic trypsinogen and does not recognize the alpha-1-proteinase inhibitor-trypsin complex. It can be used as a complement to classical immunoassays to characterize trypsinogen activation in pathological cases. The evaluation and the specificity of the assay are presented.