When the oxidative refolding of lysozyme (Lyzm) was carried out in the presence of protein disulfide isomerase (PDI) an increased refolding rate and a recovered activity exceeding 100% were reproducibly observed. The origin of this excess activity was investigated by HPLC, SDS-PAGE, and mass spectrometry and assessed using an assay for Lyzm activity. The refolding of Lyzm was achieved through the formation of PDI-Lyzm intermediates and the excess activity was derived from the nascent lysozyme released from these complexes. The released lysozyme exhibited a higher molecular activity than observed for the native protein.