Heparin proteoglycan formed by incubation of purified rat peritoneal mast cells with [35S]sulfate has previously been shown to be a proteoglycan with an approximate average molecular weight of 750,000 by gel filtration. This proteoglycan which was isolated in the absence of proteases has now been shown to contain approximately equal amounts of serine and glycine which together account for 82% of the protein. Other amino acids include 4% asparagine/aspartic acid, 3% glutamine/glutamic acid, 2% proline, valine, and lysine, and traces of alanine, threonine, methionine, histidine, and arginine. Heparin proteoglycan formed with [35S]sulfate and [3H]serine or [35S]sulfate and [3H]glycine demonstrated a polydispersity in size, with the [35S,3H]heparin proteoglycans having a higher ratio of [35S]glycosaminoglycan to [3H]protein with increasing size. This is consistent with a similar core size for large and small proteoglycan. Furthermore, fragments of [3H]serine-labelled or [3H]glycine-labeled core protein obtained after alkali treatment of both large and small proteoglycans appeared similar in size upon gel chromatography, and the [35S]glycosaminoglycan portions of the [35S,3H]heparin proteoglycan were identical in size for larger and smaller proteoglycans. As reflected by the charge characteristics, the sulfate content of the [35S]glycosaminoglycan chains derived from larger and smaller [35S,3H]proteoglycans also appeared to be the same. Thus, the principal difference between large and small [35S]heparin proteoglycan appeared to be the number of glycosaminoglycan chains per core and not their size or relative sulfation. Since changes in size of heparin proteoglycan have been shown previously to take place over several hours of synthesis, such growth might occur by an increase in the number of heparin glycosaminoglycan chains on a constant core. It was estimated that on the average, 2/3 of the serine residues of the core were substituted by heparin glycosaminoglycan chains which had an average molecular weight of approximately 75,000 for a nonsulfated chain.