A significant elevation of serum immunoglobulin (Ig) concentration occurred in female gilthead sea bream Sparus aurata during spawning. Furthermore, a progressive rise of serum Ig level was observed throughout the process of sexual inversion (from functional male to functional female), suggesting that the synthesis of Ig could be regulated by sex-related factors (probably sexual hormones) involved in the process of oogenesis. The immunoglobulins of eggs were purified by affinity chromatography on protein A-sepharose. SDS-PAGE and Western blot analysis showed reactivity of the antiserum Pab1 with the Ig heavy and light chains, and some degradation products. This purification process yielded detectable amounts of Ig. The sex-related increase of serum Ig during the reproductive period, and the detection of Ig in eggs suggest a transfer of Ig from the blood of the adult female. 2001 The Fisheries Society of the British Isles