The molecular composition of a variety of basement membranes has been intensively investigated in recent years. As a result, the list of macromolecular components present in basement membranes now includes collagenous glycoproteins (Kefalides, 1975; Burgeson et al., 1976), noncollagenous glycoproteins (Timl et al., 1979; Chung et al., 1979; Carlin et al., 1981), and glycosaminoglycans (Kanwar and Farquhar, 1979b; Lemkin and Farquhar, 1981). With regard to the latter class of macromolecular compoonents, it has been shown that the negatively charged glycosaminoglycan, heparan sulfate, is the principal glycosaminiolgycan present in the basement membrane of the renal glomerulus, a highly specialized microvascular bed (Kanwar and Farquhar, 1979a, b). This observation is of considerable importance because the presence of a polyanionic constituent, heparan sulfate, in the basement membrane significantly influences the permeability properties of the glomerular filter (Rennke et al., 1978; Kanwar et al., 1980). The findings regarding the specialized glomerular basement membrane raise the question whether anionic glycosaminoglycans are present in other vascular basement membranes where, conceivably, they may also play an important physiologic role. To begin to answer this question, we have , in this study, investigated whether we could detect the presence of heparan sulfate proteoglycan in the basement membrane of continuous capillaries. Heparan sulfate proteoglycan consists of a polypeptide core to which are linked numerous negatively charged glycosaminoglycan side chains. Heparan sulfate proteoglycan has been isolated from the matrix produced in vivo by the transplantable Engelbreth-Holm-Swarm (EHS) sarcoma (Orkin et al., 1977), and antibody to its polypeptide core has been raised in rabbits by Hassell et al. (1980). With the antibody, these investigators have localized heparan sulfate around skin and glomerular capillaries by immunofluorescence. At the light microscopic level, the antigen cannot, however, be precisely localized to the basement membrane, because the resolution of the light microscope does not allow this structure to be distinguished from closely adjacent structures. Therefore, in the present study we have used the same antibody to localize heparan sulfate proteoglycan and examine its exact distribution at the ultrastructual level in the wall of continuous capillaries of the mouse diaphragm. A preliminary description of these findings has been published earlier (Charonis et al., 1981)