The monoclonal antibody JB3 was previously shown to react with a protein antigen present in the bilateral primitive heart-forming regions and septation-stage embryonic hearts; in addition, primary axial structures at primitive streak stages are JB3-immunopositive (Wunsch et al.  Dev. Biol. 165:585-601). The JB3 antigen has an overlapping distribution pattern with fibrillin-1, and a similar molecular mass (Gallagher et al.  Dev. Dyn. 196:70-78; Wunsch et al.  Dev. Biol. 165:585-601). Here we present immunoblot and immunoprecipitation data showing that the JB3 antigen is secreted into tissue culture medium by day 10 chicken embryonic fibroblasts, from which it can be harvested using JB3-immunoaffinity chromatography. A single polypeptide (Mr = 350,000), which was not immunoreactive with an antibody to fibrillin-1, eluted from the affinity column. Mass spectroscopy peptide microsequencing determined the identity of the JB3 antigen to be an avian homologue of fibrillin-2. Live, whole-mounted, quail embryos were immunolabeled using a novel microinjection approach, and subsequently fixed. Laser scanning confocal microscopy indicated an elaborate scaffold of fibrillin-2 filaments encasing formed somites. At more caudal axial positions, discrete, punctate foci of immunofluorescent fibrillin-2 were observed; this pattern corresponded to the position of segmental plate mesoderm. Between segmental plate mesoderm and fully-formed somites, progressively longer filamentous assemblies of fibrillin-2 were observed, suggesting a developmental progression of fibrillin-2 fibril assembly across the somite-forming region of avian embryos. Extensive filaments of fibrillin-2 connect somites to the notochord. Similarly, fibrillin-2 connects the mesoderm associated with the anterior intestinal portal to the midline. Thus, fibrillin-2 fibrils are organized by a diverse group of cells of mesodermal or mesodermally derived mesenchymal origin. Fibrillin-2 microfilaments are assembled in a temporal and spatial pattern that is coincident with cranial-to-caudal segmentation, and regression of the anterior intestinal portal. Fibrillin-2 may function to impart physical stability to embryonic tissues during morphogenesis of the basic vertebrate body plan.