The core protein of a small chondroitin/dermatan sulfate proteoglycan expressed by human fibroblasts and present in extracellular matrices in association with collagen has been cloned from a lambda gt11 fibroblast cDNA library. cDNA clones were isolated by use of antibodies specific for the intact proteoglycan and antibodies against a peptide synthesized on the basis of the amino-terminal sequence of the core protein. A 1.8-kilobase cDNA was found to code for a prepro core protein composed of a signal peptide, a propeptide, and a mature core protein of 329 amino acids. The amino-terminal amino acid sequence deduced from the cDNA sequence was identical to that previously obtained by protein sequencing. The core protein contains three Ser-Gly dipeptide sequences, of which one is substituted with glycosaminoglycan. A protein data base homology search established the core protein sequence is a unique sequence distinct from published amino acid sequences. RNA blot hybridizations, performed using the cloned cDNA as a probe, revealed two related transcripts of 1.6 and 1.9 kilobases in RNA from both human fibroblast and placental tissue. Hybridization of genomic DNA restriction fragments suggested that there is one gene for the core protein of this proteoglycan and possibly one other closely related gene. Availability of the cloned cDNA for the proteoglycan now makes it possible to apply methods of molecular biology to study the collagen-binding and cell attachment-inhibiting properties of this proteoglycan.