Recombinant human leukocyte interferon produced in bacteria (IFLrA) was purified to homogeneity with the use of monoclonal antibodies against leukocyte interferon. The purified interferon exhibited a single band of Mr = approximately 19,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Amino acid analysis and the NH2-terminal sequence were consistent with the sequence predicted from the DNA. Some of the purified product contained NH2-terminal methionine; the terminal methionine was removed from the rest of the chains.