The present application relates to novel PEG-G-CSF conjugates in which a PEG molecule is linked to the cyteine residue in position 17 of native G-CSF primary sequence or to the cysteine residue of the corresponding position of a G-CSF analogue. The present application also describes a process for the manufacture of such conjugates, such process comprising the following steps: (i) subjecting the G-CSF protein to conditions inducing reversible denaturation of the protein, (ii) conjugation of the denatured protein obtained is step i with a thiol-reactive PEG under denaturing conditions, (iii) subjecting the conjugates obtained in step ii to conditions promoting renaturation of the conjugate yielding biologically active G-CSF-PEG conjugate.
