Homozygous brachymorphic (bm/bm) mice are characterized by disproportionately short stature. Newborn bm/bm epiphyseal cartilages are shorter than normal although the cells in the different zones of growth are relatively well organized. The extracellular matrix reacts poorly with stains specific for sulfated glycosaminoglycans. The ultrastructural appearance of the cartilage matrix indicates normal collagen fibrils; however, proteoglycan aggregate granules are smaller than normal and are present in reduced numbers, particularly in the columnar and hypertrophic zones of the growth plate. In addition, a prominent network of fine filaments, which are extractable in 4 M guanidine hydrochloride, are present in the bm/bm cartilage matrix. These findings suggest that a defect affecting the proteoglycan component of cartilage occurs in bm/bm mice.