Giant catfish growth hormone (gcGH) cDNA was cloned and expressed in E. coli. The expected 20.5 kDa protein corresponded to the mature gcGH and was efficiently expressed. This protein was produced as inclusion bodies and comprised about 20% of total cellular proteins. The recombinant hormone promoted growth when injected intramuscularly or intraperitoneally into goldfish (Carassius auratus) at 0.1 or 1 mug soluble gcGH per g fish body wt per week. In addition, the recombinant gcGH inclusions had growth-promoting activity similar to that of the soluble form when the fish was received either by intraperitoneal injection or by oral administration.