The type II collagenopathies form a continuous spectrum of clinical severity, ranging from lethal achondrogenesis type II and hypochondrogenesis, through spondyloepiphyseal dysplasia, spondyloepimetaphyseal dysplasia and Kniest dysplasia to the Stickler syndrome and familial precocious osteoarthropathy at the mildest end of the spectrum. We have carried out a radiographic, morphologic, biochemical and molecular study in a case of achondrogenesis type II. Electron micrographs showed inclusion bodies of dilated rough endoplasmic reticulum in the chondrocytes and the presence of sparse collagen fibers in the cartilage matrix. Protein analysis of collagen from cartilage indicated posttranslational overmodification of the major cyanogen bromide peptides, and suggested a mutation near the carboxyl terminus of the type II collagen molecule. Analysis at the DNA level demonstrated that the phenotype was produced by a single base change (G-->C) that resulted in the substitution of glycine691 by arginine in the type II collagen triple helical domain. We confirm previous observations in three cases of hypochondrogenesis that glycine substitutions in the alpha 1(II) chain can result in a phenotype at the most severe end of the type II collagenopathy spectrum.