Vitellogenin (vtg) is a high molecular weight (250 – 600 kD) glycophospholipoprotein synthesised in the liver in response to the steroid hormone 17-β estradiol (E2)(reviewed in Specker and Sullivan, 1994). It is secreted into the bloodstream from where it is taken up by developing oocytes and cleaved into the two yolk proteins, lipovitellin and phosvitin. Male fish also possess the vtg gene but expression is not normally found, due to the absence of substantial levels of circulating estrogens (Harries et al., 1997). However, in response to exogenous E2 treatment (Yao and Crim, 1996) or in response to estrogen analogues in the environment (Harries et al., 1997; Metcalfe et al., 2001), vtg secretion can increase dramatically.
   Rainbow trout were injected with either saline (one fish) or saline suspension of E2 (four fish) at a dose of 5 mg kg-1 body weight. Four greenback flounder and four Atlantic salmon, were each divided into two equal treatment groups and injected with either saline, or a saline suspension of E2 at a dose of 2 mg kg-1 body weight. Atlantic salmon were injected twice at an interval of one week, and rainbow trout and greenback flounder were injected three times at weekly intervals. Injection volume was 1ml kg-1 body weight for all three species.
   Gel filtration demonstrated that plasma from male fish from all three species that had been inoculated with E2 contained a high level of protein with a molecular weight close to that described for vtg in other species (fig 1) (Nath, 1999; Yao and Crim, 1996). This peak was absent from saline-injected fish indicating that this protein fraction was E2-inducible and therefore reasonably identifiable as putative vtg.

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