The messenger RNAs and core proteins of the two small chondroitin/dermatan sulfate proteoglycans, biglycan and decorin, were localized in developing human bone and other tissues by both 35S-labeled RNA probes and antibodies directed against synthetic peptides corresponding to nonhomologous regions of the two core proteins. Biglycan and decorin expression and localization were substantially divergent and sometimes mutually exclusive. In developing bones, spatially restricted patterns of gene expression and/or matrix localization of the two proteoglycans were identified in articular regions, epiphyseal cartilage, vascular canals, subperichondral regions, and periosteum, and indicated the association of each molecule with specific developmental events at specific sites. Study of non-skeletal tissues revealed that decorin was associated with all major type I (and type II) collagen-rich connective tissues. Conversely, biglycan was expressed and localized in a range of specialized cell types, including connective tissue (skeletal myofibers, endothelial cells) and epithelial cells (differentiating keratinocytes, renal tubular epithelia). Biglycan core protein was localized at the cell surface of certain cell types (e.g., keratinocytes). Whereas the distribution of decorin was consistent with matrix-centered functions, possibly related to regulation of growth of collagen fibers, the distribution of biglycan pointed to other function(s), perhaps related to cell regulation.