A method for inactivating enzymes, particularly enzymes found in human serum. The method comprises reacting the enzyme in a suitable medium, e.g. human serum, with an inactivating amount of peracetic acid. In the preferred embodiment the treated serum is subsequently neutralized and dialyzed. Preferably, the serums are used as standards for calibration or as a control in assay kits, e.g. RIA, and EIA or enzyme kits, for the determination of specific ligands and enzymes.