We have performed histochemical, immunohistochemical, electron microscopic, and biochemical studies on the upper tibial cartilage from a case of homozygous achondroplasia. The growth zone was narrow and disorganized. Columnization was absent except for a few areas with short rows of cells. Hypertrophy was reduced to scattered clusters of cells. The provisional calcification was patchy and primary trabeculae were thick and irregularly arranged. Islands of fibrous or fibrocartilagineous tissue were found along the growth zone. The matrix did not stain with safranin O and lacked metachromasia, except for pericellular rims around the hypertrophic cell clusters. Staining with antibodies against the large proteoglycan monomers and chondroitin-4-sulfate was weakly positive. Electron microscopic examination showed that only a few cells had degenerative signs. In most areas of the matrix, proteoglycan granules were absent. Areas with dense collagen fibers were seen. In contrast to the growth zone, the cartilage of the remaining epiphyses had normal histochemical, immunohistochemical, and electron microscopic appearance. The large proteoglycan monomers had a normal composition and hydrodynamic size. Type II and XI collagen, pepsin fragments of type IX collagen, and several noncollagenous proteins extracted from cartilage had a normal electrophoretic migration. It is suggested that a mutation affecting a matrix component or a regulatory pathway present only or predominantly in the growth area of the chondroepiphysis might explain the findings.