PURPOSE: To obtain a protein, such as recombinant somathrombin, having the activity to promote the growth of animals, in a high yield, by treating a reduced recombinant protein in a denaturant soln. with cystine, oxidizing free sulfhydryl group and forming an intramolecular sulfide bond.

CONSTITUTION: The denaturant soln. contg. sodium dodeculsulfate(SDS), etc., in an amt. of 0.05 to 5 wt.% is added and dissolved to and in the reduced recombinant protein (e.g.: reduction type somathrombin, etc.), produced by culturing coliform bacilli, etc., subjected to transformation by a recombinant gene and the cystine is added thereto in an amt. of 0.5 to 10 mol. for each 1 mol. of the recombinant protein. The pH of the soln. is maintained at 7 to 1 and the reaction is effected for 6 to 48 hours to oxidized the free sulfhydryl of the recombinant protein and to form the intermolecular disulfide bond, to obtain the recombinant protein having bioactivity.