Cartilage oligomeric matrix protein (COMP), the fifth member of the -thrombospondin gene family, is an extracellular matrix calcium-binding protein. The importance of COMP is underscored by the finding that mutations in COMP cause the human dwarfing condition, pseudoachondroplasia (PSACH). Here, we report the results of human tissue distribution and cell secretion studies of human COMP. COMP is expressed and secreted by cultured monolayer chondrocyte, tendon and ligament cells, and COMP secretion is not restricted to a differentiated chondrocyte phenotype. Whereas COMP is retained in the endoplasmic reticulum that accumulates within PSACH chondrocytes in vivo, COMP is not retained intracellularly in the dedifferentiated PSACH chondrocytes in cultures. These results lend further support to the hypothesis that retention of COMP is related to the terminal PSACH chondrocyte phenotype, processing of proteins related to extracellular matrix formation, and maintenance in cartilage.