The Swarm chondrosarcoma, a transplantable tumor maintained in rats, has a structure similar to hyaline cartilage. The extracellular matrix contains narrow, banded collagen fibrils and proteoglycan matrix granules. The cells are filled with organelles typically involved in synthesis and secretion, including an extensive rough endoplasmic reticulum with dilated cisternae, a large Golgi apparatus dispersed throughout the cell, and secretory vacuoles which fill directly from Golgi saccules. There is only one class of secretory vacuoles, suggesting that the secretory products, collagen and proteoglycan, are packaged and secreted together. The cells also contain unusual endoplasmic reticulum, many coated vesicles and multivesicular bodies, and abnormal mitochondria. The large amount of tissue available from the tumor, and its characterization biochemically (Choi et al., '71; Oegema et al., '75) and morphologically as cartilage, make the tumor valuable as a model tissue for studies of synthesis and secretion of extracellular matrix materials.