Abstract: Muteins of IL-2 in which one of the amino acids of the mature native sequence of IL-2 is replaced by a cysteine residue are prepared and conjugated through the replaced cysteine residue to a polymer selected from polyethylene glycol homopolymers or polyoxyethylated polyols, wherein said homopolymers are unsubstituted or substituted at one end with an alkyl group. These muteins are made via host expression of mutant genes encoding the muteins that have been changed from the genes for the parent proteins by site-directed mutagenesis. In addition, other species of IL-2 may be conjugated via the cysteine residue at position 125 of the mature IL-2 protein that is not necessary for the biological activity of the IL-2.