A protein responsible for the protective and reactivating activities of two active fractions (AF1 and AF2) of the cells of Propionibacterium freudenreichii subsp. shermanii was isolated. The active fraction AF1 was obtained by fractional precipitation of the cell-free extract of propionic acid bacteria between 20 and 40% ammonium sulfate saturation, whereas fraction AF2 was precipitated between 60 and 80% saturation. Further fractionation of AF1 and AF2 by gel filtration on Sephacryl S-200 and by ion-exchange chromatography on DEAE-Sepharose yielded seven active subfractions, as revealed by testing for their protective activity on UV-inactivated cells of Escherichia coli. Analysis of subfraction AF2-2.5 by SDS-electrophoresis and HPLC showed that it contained an apparently homogeneous protein with a molecular mass of 44 +/- 2 kDa. The concentrational dependence of the protective activity of this protein was derived. Peptides of subfractions AF2-2.1 and AF2-2.2 with molecular masses lower than 15 kDa also exhibited protective activity.