1. Vitellogenin was purified from plasma of estrogen-treated spotted seatrout (Cynoscion nebulosus) and was shown to be a protein of approximately 200,000 mol. wt. 2. Antisera were raised against the isolated protein and were used to develop a radioimmunoassay for spotted seatrout vitellogenin. 3. The substance measured was shown to be female-specific, hepatically-derived, estrogen-inducible and immunologically related to oocyte yolk proteins. 4. Vitellogenin levels in the plasma of female spotted seatrout were shown to rise dramatically during the period of ovarian recrudescence.