Monomethoxypolyethylene glycol (mPEG) of average molecular weight 5000 was transformed in a series of synthetic steps to a new activated form of PEG, a stable thiol-protected intermediary, for reaction with cysteine residues in proteins under mild conditions to produce PEG--protein conjugates as possible candidates for therapeutics. The modified protein has PEG polymer molecules attached to the backbone by the newly formed disulfide bonds, which are readily cleaved to regenerate the native protein under mild reducing conditions. The model protein papain, which has seven cysteine residues including a lone cysteine in its active site, was modified through the free available thiol. The resulting PEG--papain was characterized by matrix-assisted laser desorption mass spectrometry, SDS-PAGE, and high performance gel filtration chromatography. The major modified PEG--papain variant was demonstrated to be 5000 Da larger than the unreacted papain.