Prolactin (PRL) is a polypeptide hormone, which in all vertebrates except Cyclostoms is synthesized in the adenohypophysis (Schriebman, 1986). PRL belongs to a family of structurally and functionally related hormones which include growth hormone, placenta lactogen, proliferin in mammals and somatolactin in fish. Based on amino acid sequence homologies, it has been reported that these proteins have arisen by duplication of an ancestral gene (Bewley & Li, 1971; Takayama et al., 1991). Although in man, rat and turkey, PRL is the product of a single gene, the hormone occurs in multiple molecular forms, cleaved, phosphorylated or glycosylated whose discrete functions remain a matter of debate (Lewis et al., 1984; Clapp et al., 1989; Brooks et al., 1990). Among teleost fish, PRL was isolated in the tilapia species Oreochromis mossambicus (Specker et al., 1985), salmonids (Idler, Shamsuzzaman & Burton, 1978; Kawauchi et al., 1983; Prunet & Houdebine, 1984; Anderson, Skibeli & Kautvik, 1989), carp (Yasuda et al., 1987) and eel (Suzuki et al., 1991). Amino acid sequence identity between fish and mammalian PRLs is only 20–30% whereas it increases to 60–80% between fish PRLs. The major difference between fish and mammalian PRLs is the absence of one disulfide loop in the N-terminal region. Nucleotide and polypeptide sequences analysis of these fish PRLs revealed the existence of two distinct, albeit similar, genes in some species. This is the case for salmon or carp (Yasuda, Itoh & Kawauchi, 1986, 1987). In tilapia species, Oreochromis mossambicus and O. niloticus, a somewhat different situation was described, and two much less similar PRL molecules were characterized (Specker et al., 1985; Yamaguchi et al., 1988; Rentier-Delrue et al., 1989).