Human Interleukin-2 (IL-2) posessing a serine in place of cystein at the amino acid sequence position 125 has been produced in E. coli by site directed mutagenesis. The activity of purified Ser^(125)-IL-2 was more than 2.5 times higher than that of Cys^(125)-IL-2 as determined by the growth promoting effect on IL-2 dependent cell line. The yield of production of Ser^(125)-IL-2 was at least 1.5 times higher as compared to that of Cys^(125)-IL-2.