Abstract: Derivatives of naturally occurring G-CSF having at least one of the biological properties of naturally occurring G-CSF, and a solution stability of at least 35% at 5 mg/ml are disclosed in which the derivative has at least Cys.sup.17 of the native sequence replaced by a Ser.sup.17 residue and Asp.sup.27 of the native sequence replaced by a Ser.sup.17 residue. Nucleotide sequences coding for part or all of the amino acid sequence of the derivatives of the invention may be incorporated into autonomously replicating plasmid or viral vectors employed to transform or transfect suitable procaryotic or eucaryotic host cells such as bacteria, yeast or vertebrate cells in culture.