Updated project metadata. We conducted immuno-proteomic investigations of recombinant and native procathepsins ((r)FhpCL1) and counterpart polyclonal antibodies to a recombinant mutant procathepsin L (anti-rFhΔpCL1). Proteomics including gel electrophoresis and liquid chromatography tandem mass spectrometry were used for formal identification and in the determination of sample compositions. We have confirmed recombinant and native cathepsin L zymogens contain conserved, highly antigenic epitopes that are conformationally dependent.