During purification of recombinant Interleukin-2 (rIL-2) by reversed-phase HPLC, early fractions are discarded due to the presence of an unidentified form of rIL-2. A procedure has been developed to isolate and purify this unidentified form of rIL-2. The purification process involves two chromatography steps and utilizes a Bakerbond Carboxy-Sulfon (CS) column under two different conditions. This material, designated as a high-molecular-weight form of rIL-2 (HMWrIL-2), exhibits lower mobility during SDS-PAGE and has a pI which is approximately one unit less than that of rIL-2, but has similar bioactivity to rIL-2. Structural analysis through enzymatic cleavage, HPLC peptide mapping, mass spectrometry, sequencing, and amino acid composition revealed that the difference between these two proteins is a C-terminal extension of 11 amino acids. This extension could be the result of a nonstandard translation event.