A murine monoclonal antibody (3B3) has been produced with specificity for chondroitin-6-sulfate (C-6-S) and proven binding to rodent basement membranes, presumably detecting a population of C-6-S-containing proteoglycans. Utilizing this antibody, we sought to determine whether a basement membrane chondroitin sulfate proteoglycan is present in adult, neonatal, and/or fetal skin, and if present, its ultrastructural localization. Indirect immunofluorescence was performed on human adult, neonatal, and fetal skin. To detect the antigen, specimens were pretreated with chondroitinase ABC; absence of enzyme treatment served as negative control. Chondroitin sulfate proteoglycan was detectable in linear homogeneous array along the dermoepidermal junction and within vascular (and when present, adnexal) basement membranes in both adult and neonatal skin. In fetal skin, basement membrane staining was noted as early as 54 gestational days. Indirect immunoelectron microscopy and NaCl-split skin studies were performed to ultrastructurally localize the antigen; immune deposits were detectable within the lamina densa in chondroitinase-treated skin. These findings demonstrate that chondroitin sulfate proteoglycan is present within all skin basement membranes; that it is present in the region of the lamina densa; and that similar to some other ubiquitous basement membrane antigens, it is present early in the developing fetus.