The invention concerns a new mutant of .gamma.-IFN. This new polypep-tide contains 134 amino acids. Amino acids 1 to 132 are the same as those of natural .gamma.-interferon. The first amino acid, methionine, in the zero posi-tion is also present, as has been demonstrated by protein sequencing. The amino acid in position 133 is leucine instead of glutamine. The invention also concerns DNA sequences and plasmid DNA (DSM 6238) which codes for this new polypeptide. The invention further concerns the use of the polypeptide as a drug as well as its use as a fine-chemical reagent for in vitro experiments.