A modified polypeptide having human granulocyte colony stimulating factor (hG-CSF) activity comprising a polypeptide having hG-CSF activity with at least one amino group thereof substituted with a group of the formula
wherein R 1 is an alkyl or alkanoyl group; n is an optionally variable positive integer; X is 0, NH or S; R 2 is
[where R 3 is OH, Cl, O-(CH 2 CH 2 O) n -R 1 , where R 1 and n are as defined above, Y may not be present or represents Z-(CH 2 )pCO, where Z is 0, S or NH and p is an optionally variable positive interger], or (CO) m -(CH 2 )gCO, where m is 0 or 1; t is an optionally variable positive integer, which can provide an enhanced peripheral leukocyte (granulocyte) increasing effect with improved stability and residence time in the blood and, as such, can be used advantageously in clinical medicines.