Gut tissue from the tunicate Pyura pachydermatina (sea tulip) was found to contain a compound or compounds which react with anti-porcine insulin antibodies, but not anti-hag-fish insulin antibodies, and which also stimulate lipogenesis in isolated rat fat cells. The insulin-like material is present in two immunologically active forms, a species of Mr 6000 apparently similar to mammalian insulin, and a high Mr form which expresses biological activity only after further purification by reverse-phase HPLC. The bioactivity of both species is suppressed in the presence of anti-porcine insulin antibodies.