PURPOSE: To obtain a new prolactin having industrial advantage such as application to growth promoter in culture of fishes, showing action related to growth promotion of marine fishes, from prolactin derived from hypophysis of Acipenser mikadoi.

CONSTITUTION: Hypophysis is extracted from Acipenser mikadoi before egg laying, freeze-dried, de-fatted, ground by a homogenizer in 35 % hydrochloric acid-acetone solution at 4°C under cooling with ice and extracted with acid- acetone for one hour on ice. The extracted solution is ultracentrifuged to separate precipitate. The precipitate is dissolved in 1M acetic acid, subjected to gel filtration, its active fraction is purified by reversed phase high performance liquid chromatography and eluted with acetonitrile-containing 0.1% trifluoroacetic acid solution. The prepared fifth fraction is lyophilized to give the objective new prolactin which has about 23kD molecular weight measured by SDS-polyacrylamide gel electrophoresis and specific amino acid constitution obtained by hydrolysis with 0.6% phenol-containing 6N-hydrochloric acid and determination.