There are disclosed chemically active poly(ethylene glycols) and other hydrophilic polymers that are suitable for coupling to pharmaceutically or diagnostically active agents such as peptides, oligonucleotides, proteins or non- peptide molecules. The compounds are represented by the formula Poly-(X-NH-CO-A) n wherein: Poly is a hydrophilic polymer having a molecular weight of from about 300 to 100000 Daltons; A together with -NH-CO- forms a reactive group; X is a spacer moiety or a bond; n is an integer comprised between 1 and 50. The active agents of interest which may be conjugated to the disclosed compounds may be selected from hemoglobin, insulin, urokinase, alpha- interferon, G-CSF, hGH, asparaginase, adenosine deaminase, superoxide dismutase and catalase.