A method of preparing a modified granulocyte colony stimulating factor (G-CSF) protein having reduced immunogenicity relative to human G-CSF comprises the steps of (i) identifying one or more potential T-cell epitopes within the amino acid sequence of human G-CSF (SEQ ID NO: 1); (ii) designing at least one sequence variant of at least one potential T-cell epitope identified in step (i), wherein the sequence variant eliminates or substantially reduces the MHC class II binding activity of the potential T-cell epitope; (iii) preparing, by recombinant DNA techniques, at least one modified G-CSF protein including a sequence variant designed in step (ii); (iv) evaluating at least one modified G-CSF protein prepared in step (iii) for G-CSF activity and immunogenicity; and (v) selecting a modified G-CSF protein evaluated in step (iv) that has substantially the same therapeutic G-CSF biological activity as, but substantially less immunogenicity than, human G-CSF.