The absence of columns of proliferative chondrocytes in mice with hereditary chondrodysplasia (cho/cho) has been attributed to a lack of structural integrity of the cartilage matrix. To determine whether the abnormality is related to defective interaction between proteoglycan and collagen, sternal cartilage from control and mutant fetuses at eighteen days of gestation was examined with autoradiographic and ultrastructural methods. The sulphate-labeling pattern in the mutant fetuses was normal, suggesting that the defect is not due to regional differences in proteoglycan synthesis. The presence of ruthenium red-stained matrix granules precipitated one collagen fibrils suggests that proteoglycan is capable of interacting with collagen in the mutant's matrix. The absence of a pericellular space and the presence of collagen fibrils adjacent to the external surface of the cell membrane suggests that the defect is due to precocious assembly of collagen monomers into fibrils, resulting in the absence of a normal network of interconnecting collagen fibrils and proteoglycan. Further studies are necessary to test this hypothesis. CLINICAL RELEVANCE: Because the phenotype of this mutation resembles certain forms of human skeletal dysplasia, mice with hereditary chondrodysplasia may serve as a model to elucidate the molecular mechanism for certain recessive disorders of chondrogenesis. In this regard, a much improved understanding of gene action is needed before effective treatment and corrective procedures can be applied to the eighty or more clinical forms of hereditary skeletal dysplasia.