Achondrogenesis type I is a perinatally lethal, short-limb chondrodysplasia. Two types, IA and IB, have been distinguished by radiographic and histological criteria; both types appear to be inherited as autosomal recessive traits. The underlying molecular defects are not known, but histochemical studies have suggested that in achondrogenesis type IB, cartilage matrix is deficient in sulfated proteoglycans. We have studied cartilage extracts of one newborn with achondrogenesis type IB and found that proteoglycans were quantitatively reduced, and, unlike in control cartilage, they did not stain with toluidine blue and did not bind to DEAE. Impaired synthesis of sulfated proteoglycans was observed also in fibroblast cultures of the achondrogenesis IB patient. Radioactive labeling and immunoprecipitation studies indicated that core protein and side chains of proteoglycans were synthesized normally but were not sulfated. Analysis of sulfate metabolism in fibroblast cultures showed, in the patient's cells, normal intracellular levels of free sulfate but markedly reduced levels of the two intermediate compounds in the sulfate activation pathway, adenosine-phosphosulfate and phosphoadenosine-phosphosulfate. The results can be explained by deficient activity of one of the enzymes responsible for the biologic activation of sulfate, possibly similar to that observed in cartilage (but not in skin) of the recessive, nonlethal mouse mutant brachymorphic and leading to defective sulfation of macromolecules. Expression of the sulfation defect in cultured fibroblasts may offer a diagnostic tool for the disorder.