Modified forms of insulin-like growth factor (IGF) are provided which demonstrate improved pharmacological and biological properties. These modifications include IGF muteins produced by substituting or adding a cysteine in the amino acid sequence of native IGF as well as such muteins attached to polyethylene glycol (PEG) at the free cysteine site. The present invention further provides methods of making such modified forms. The IGF-PEG conjugates can be formulated into pharmaceutical compositions and used for the therapeutic treatment of IGF associated conditions.
