Cellular mechanisms of Ig transfer to Sparus aurata developing oocytes were analysed. Homologous serum Ig was purified and used to raise the rabbit polyclonal antiserum ORa. Immunohistochemistry revealed an active role of both follicular cells (already at a pre-vitellogenetic stage) and oocytes in the Ig uptake. Early vitellogenetic oocytes (lipid vesicle stages) had ORa staining of outer cortex and oolemma as well as of their follicular cells. In protein yolk granule oocytes, ORa staining was notably found in the pore canals crossing the egg envelope and at the periphery of yolk platelets. A transfer of Ig from the blood to the follicles appears likely. In addition, RT-PCR using specific primers for the constant region of sea bream Ig L chain detected Ig mRNA in released eggs and no signal in post-hatching larvae. These findings show that a significant level of Ig gene transcription in the oocyte and/or a transfer of transcripts may also occur.