Cultured bovine aortic smooth muscle (BASM) and endothelial (BAE) cells, like articular chondrocytes, synthesize two populations of small chondroitin/dermatan sulfate (CS/DS) proteoglycans (PGs) of similar size as PG-I/biglycan and PG-II/decorin. However, Northern blot analyses demonstrated that BAE cells express detectable amounts of mRNA transcripts only for PG-I/biglycan, whereas BASM cells and articular chondrocytes express mRNA transcripts for both PG-I/biglycan and PG-II/decorin. Endothelial cells from human umbilical vein also expressed detectable amounts of mRNA transcripts only for PG-I/biglycan, and not PG-II/decorin. Antiserum raised against bovine PG-II/decorin immunoprecipitated an apparent single PG species with relative molecular mass (Mr) of approximately 120,000-180,000 from BASM cell and articular chondrocyte cultures but failed to immunoprecipitate an equivalent PG species from BAE cell cultures, consistent with the results from Northern blot analysis. In contrast, immunoprecipitations by antisera to PG-I/biglycan indicated that cultured endothelial cells synthesize two forms of PG-I/biglycan with Mr values slightly larger than 200,000 and 120,000-140,000. It is likely, based on the magnitude of the size difference, that these two forms of PG-I/biglycan differ in the number of glycosaminoglycan chains. Additionally, BASM but not BAE cells were found to express detectable amounts of mRNA transcripts for type I collagen. The above results indicate that the two main cell types of the vascular wall, endothelial and smooth muscle cells, express different sets of small interstitial CS/DS PGs and that the synthesis of PG-II/decorin by these cells correlates with the expression of type I collagen, a collagen known to interact specifically with this PG. These differences in the expression of extracellular matrix molecules may be important in regulating the cell type-specific functions of endothelial and smooth muscle cells within the vascular tissue.