The Wistar Furth (WF) rat has a hereditary defect in platelet formation that resembles gray platelet syndrome of man with a large mean platelet volume and platelet alpha granule deficiency. The alpha granule abnormality is suggestive of a defect in granule packaging and/or stability. Proteoglycans are hypothesized to play a role in granule packaging. Therefore, we have analyzed the structure of the platelet proteoglycan, serglycin, in platelets of WF and normal Wistar rats. Normal and Wistar Furth rats were injected with 35S-sulfate to label platelet proteoglycans via synthesis by the megakaryocytes, and platelets were isolated 3 days later. We found that WF rat platelets have only one-third of the normal proteoglycan mass per unit platelet volume, and the proteoglycans are smaller in hydrodynamic size with shorter glycosaminoglycan chains than those of Wistar rats. However, WF rat platelet proteoglycans showed no defect in binding to collagen on affinity coelectrophoresis gels. We conclude that the structure of WF rat platelet proteoglycans is abnormal, and speculate that this abnormality may contribute to abnormal packaging of the alpha granule contents. Leakage of alpha granule contents into the marrow by platelets and megakaryocytes could perturb the marrow matrix, and promote the development of myelofibrosis noted in gray platelet syndrome.