The complete nucleotide sequence of a mouse mastocytoma proteoglycan core protein mRNA was determined. The mRNA, estimated to contain 1.1 kb, encodes a protein with an Mr of 16715. A 21-amino acid-residue region of the protein is composed of alternating serine and glycine residues. Southern-blot analysis of mouse genomic DNA with cDNA containing sequences corresponding to the Ser-Gly repeat region revealed more than 15 gene fragments. Hybridization with a probe corresponding to the N-terminal portion of the core protein identified two fragments, and cDNA covering the C-terminal part of the core protein and the 3' untranslated part of the mRNA hybridized to a single fragment. Antibodies against the core protein, obtained after immunization of rabbits with a fusion protein, reacted with both chondroitin sulphate proteoglycans and heparin proteoglycans produced by the tumour. In immunoblotting of a microsomal fraction from the mastocytoma, the antiserum recognized a single protein (Mr 17,000), which probably represents the core protein before glycosylation.