Chicken embryos were immunolabeled with antibodies to fibrillin, an extracellular matrix molecule implicated in the Marfan syndrome. The pattern of labeling, established using both whole-mounted and sectioned embryos, showed a striking localization at the primary axis of the early embryo, including Hensen's node. After stage 8, the labeling of Hensen's node diminished while other structures in the embryo exhibited increased immunolabeling. These structures include the region of the presumptive dorsal aorta, the ventral surface of the notochord, and the mesocardium. Injections of the antibody between the vitelline membrane and the blastoderm in vivo failed to perturb the development of embryos, thus precluding any conclusions regarding the function of fibrillin.