A modified human interferon-alpha 2 was produced in Escherichia coli cells infected with phage M13 mp7 containing an interferon-alpha gene. After purification by immunochromatography with the monoclonal antibody NK2, the N-terminal amino acid sequence was determined. The N-terminal methionine was absent but an additional sequence of 18 amino acids at the N-terminus was retained. The modified interferon-alpha 2 was indistinguishable from authentic interferon-alpha 2 in its ability to activate natural killer cells, to slow the growth of Daudi cells, and to confer virus resistance on heterologous cells.