The potential absorptive role of the yolk sac membrane was evaluated by examining protein and enzyme patterns in embryonic fluids and by comparing the synthetic capacity of the secondary yolk sac, fetal liver and placenta for human chorionic gonadotrophin (HCG) and alpha-fetoprotein (alphaFP). In yolk sac fluid samples, protein electrophoresis showed two main electrophoretic bands with mobilities comparable to those of albumin and interalbumin-alpha1-globulin, and immunoblotting revealed the presence of albumin, alphaFP, alpha1-antitrypsin, alpha2-macroglobulin, transferrin, complement factors 3 and 4 and immunoglobulin G. In coelomic fluid, similar results were obtained, except for the absence of alpha2-macroglobulin and the presence of ceruloplasmin and IgA. After electrophoresis and immunoblotting with specific antibodies, beta-HCG was detected in all placental homogenates and culture media but was not revealed in any of the corresponding yolk sac tissue samples. Reverse transcription-polymerase chain reaction (RT-PCR) showed that all placental samples express beta-HCG mRNA whereas all yolk sac and liver samples express alphaFP mRNA. These findings suggest that the yolk sac membrane is an important zone of transfer between the extra-embryonic and embryonic compartments and may also help to further develop therapeutic protocols making use of fetal somatic gene therapy by injecting transduced cells into the exocoelomic cavity.