A mutated recombinant human interferon- beta protein (I) in which an amino acid having at least one hydroxy group is substituted for at least one of Leu5, Phe8, Phe15, Leu47, Phe50, Leu106, Phe111, Leu116, Leu120 and Phe156, is new. USE - (I) is used in medicaments e.g. for treating multiple sclerosis, for in vitro screening assays and for measurement of interferon levels (claimed). ADVANTAGE - The mutated protein is more water-soluble than recombinant wild-type human interferon- beta .