We have determined the primary structure of a connective tissue matrix protein from the nucleotide sequence of a clone isolated from a human articular chondrocyte cDNA library. The major part of the amino acid sequence has also been determined by direct protein sequencing. The translated primary sequence corresponds to 382 amino acid residues, including a 20-residue signal peptide. The molecular mass of the mature protein is 41,646 Da. The main part of the protein consists of 10 leucine-rich repeats ranging in length from 20 to 26 residues, with asparagine at position 10 (B-type). The N-terminal part is unusual in that it is basic and rich in arginine and proline. There are four potential N-linked glycosylation sites present. In three of these sites, post-translational modifications are likely to be present since Asn was not found by direct protein sequencing. The amino- and carboxyl-terminal parts contain four and two cysteine residues, respectively, probably forming disulfide bonds by analogy with the other members of this family. The protein shows highest identity (36%) to fibromodulin and 33% to bovine lumican, two other leucine-rich repeat connective tissue proteins. Northern blot analysis showed the presence of an approximately 3.8-kilobase mRNA in different types of bovine cartilage and cultured osteoblasts, whereas RNAs isolated from bovine kidney, skin, spleen, thymus, and trabecular bone and rat calvaria were negative. Human articular chondrocyte and rat chondrosarcoma cell RNAs contained an additional mRNA of approximately 1.6 and 1.8 kilobases, respectively.