The recombinant human interferon alpha 2a ($rhIFN-{alpha}2a$), expressed in Saccharomyces cerevtsiae, was purified from insoluble aggregates. The inclusion body of $rhIFN-{alpha}$ was solubilized by guanidine salt in the presence of disulfide reducing agent. The refolding of denatured $rhIFN-{alpha}2a$ was achieved by simple dilution. The authentic interferon alpha, which has two correctly matched disulfide bonds, was seperated from incompletely oxidized $IFN-{alpha}$ and dimeric $IFN-{alpha}$ by use of a CM-Sepharose column, followed by size exclusion columns at two different pH conditions. The purified protein has been subjected to detailed physicochemical characterization including sequence determination. Unlike other $rhIFN-{alpha}2a$ from E. coli reported, the $rhIFN-{alpha}2a$ from S. cerevisiae has no methionine residue at its N-terminus originating from the start codon, ATG. The pI of the protein was determined to be 6.05 with a single band in the pI gel, which demonstrated that the purified $rhIFN-{alpha}$ was homogeneous. The structural study using circular dichroism showed that the protein retains its three dimensional structure in the wide range of pH conditions between pH 3 and 9, and only minor strucural deformation was observed at pH 1.0.