Benzoxazolon-3-yt-acetic acid was esterified with diethylene glycol, triethylene glycol and polyethylene glycols with Mw in the range of 200 1000 D by means of the dicyclohexylcarbodiimide procedure in tetrahydrofuran, resulting in the corresponding monoesters in comparatively good yields (70-92%). The products were characterized by ~H-NMR, i.r. and u.v.-spectra as well as by GPC, VPO and DSC methods. The relatively narrow molecular weight distribution was preserved in the process of the chemical modification. The degree of esterification is gradually decreasing from 91% to 44% with the increase of the length of the polyether chain. The synthesized conjugates are homogeneous as judged by GPC analysis, but clear evidence for heterogeneity is obtained by the more sensitive HPLC technique. The molecular mass of the polyethylene glycols used affects the physicochemical properties thus providing 2-benzoxazolon-3-yl-acetic acid polyethylene glycol conjugates of variable properties able to meet different kind of demands. We found that the conjugate with PEG400 is extremely useful as a substrate in the penicillin amidase catalysed transfer of 2-benzoxazolon-3-yl-acetyl moiety on 7-aminodesacetoxycephalosporanic acid offering a new cephem in high yield (70%).