1. Injection of oestradiol results in induction of vitellogenic proteins in the plasma of stickleback, sea bass, sea horse, pipe-fish, rainbow trout and roach. 2. Immunoblotting after SDS-PAA gradient gel electrophoresis of plasma proteins of stickleback, sea bass and rainbow trout reveals that vitellogenins from these species share common antigenic determinants. 3. Yolk proteins are recognized by antivitellogenin antisera which shows that proteolytical cleavage does not destroy all antigenic determinants. 4. Vitellogenic proteins are present only in the yolk granules and near the theca cell layer as is shown by immunocytochemical methods.