A method for industrially producing human activin A which comprises refolding a modified human activin A produced by a microorganism into natural-form human activin A having a biological activity. This method comprises the steps of: (a) solubilizing the modified human activin A with a modifier and then protecting the thiol groups with glutathione and/or sodium sulfite; and (b) dialyzing the protected modified human activin A with a refolding buffer and then adding a thiol compound thereto to conduct a disulfide bond interchange reaction.
