Poly(2-ethyl 2-oxazoline) (PEOZ) is a water-soluble, stable and biocompatible polymer that was prepared in a linear form for the conjugation of protein biomolecules. Polymers of molecular weights ranging from 5 to 20kDa, with an aldehyde or an amine functional terminal group, were synthesized with narrow polydispersities. To assess the suitability of the polymer for therapeutic application, granulocyte colony stimulating factor (G-CSF) was used as a model protein for PEOZ conjugation. Two coupling strategies were employed, namely the chemical N-terminal reductive amination and the enzymatic transglutaminase (TGase) mediated glutamine conjugation. The secondary structure of the protein, measured by circular dichroism, was maintained upon PEOZylation and the stability of conjugates toward aggregation at 37°C was improved compared to G-CSF. The potency of PEOZ-G-CSF mono-conjugates was tested in vitro by cell proliferation assays and in vivo by studying the effects on white blood cell and neutrophil count increases in normal rats. The results have shown that PEOZ is suitable for protein conjugation by both chemical and enzymatic methods and that the conjugates of G-CSF retained high biological activity, both in vitro and in vivo. Display omitted