We have modified recombinant interleukin-2 (rIL-2) to facilitate site-directed covalent attachment of monomethoxy polyethylene glycol (PEG). The site chosen for modification and subsequent covalent attachment with PEG (PEGylation) was the single glycosylation position found in the native interleukin-2 (IL-2). The mutant protein was expressed in E. coli, purified, and PEGylated with a PEG-maleimide reagent to obtain PEG-cys3-rIL-2. The PEG-cys3-rIL-2 had full bioactivity relative to the unmodified molecule and had an increase in hydrodynamic size sufficient to increase its systemic exposure by approximately 4 fold. This method has general applicability for modifying any therapeutic protein at a specific site and thereby alter its potency. In particular, it can be used to attach PEG to prokaryotically expressed recombinant proteins at their glycosylation sites.