A portion of the recombinant interleukin-2 (rIL-2) produced in Escherichia coli had an amino (N-) terminal methionine. We found that this partial processing of the N-terminal methionine was caused by the proline residue, which is the second amino acid of IL-2. Alanine or histidine was substituted for proline by site-directed mutagenesis. There were no marked differences between each mutein and rIL-2 in the amount of product, accumulation states or bioactivity. However, these muteins had no additional methionine at the amino terminus.