We describe here an antigen marker, designated VE-1, that is detected early in gastrulation (approximately E6.5 through approximately E7.25) in the anterior visceral endoderm overlying the embryonic ectoderm opposite the primitive streak. The antibody-positive domain extends from the embryonic-extraembryonic junction to the distal tip of the embryo, and laterally around approximately one-third of the circumference of the egg cylinder. Analysis of embryos at earlier stages indicates that VE-1 is first expressed shortly after implantation, at approximately E5.0, in the visceral endoderm on one side of the embryo and thus is the earliest molecular marker of A-P asymmetry in the post-implantation mouse embryo described to date. Although VE-1 was detected with a polyclonal antiserum raised against a 24 amino acid polypeptide sequence of FGF2, we provide evidence that the VE-1 antigen is not FGF2. The data reported here are the first to provide molecular evidence that A-P polarity in the mouse embryo is established by E5.0 and that the visceral endoderm has A-P polarity.