Matrix vesicles were identified in the cartilage matrix of the upper tibial epiphyseal plate of normal mice. These vesicles were found throughout the plate and were not typically in contact with cartilage cells. They were concentrated in the longitudinal septa from the proliferative zone downward, varying in size (300 Å to 1 μm) and shape. The vesicles were bounded by unit membranes and contained materials of varying density, including rare ribosomes. A close association was found between matrix vesicles and calcification, with vesicles in juxtaposition to needle-like structures identified as hydroxyapatite and/or fluorapatite. Decalcification revealed similar structures in previously calcified cartilage. The study suggests that matrix vesicles may originate from cells and play a role in initiating calcification at the epiphysis. The vesicles were observed in various regions of the epiphyseal plate, with a characteristic distribution. Their fine structure varied, and they were not cytoplasmic projections from cartilage cells. The presence of matrix vesicles in the matrix of calcified cartilage and their association with apatite deposition suggest a role in calcification. Electron diffraction data supported the identification of hydroxyapatite and fluorapatite. The study concludes that matrix vesicles may be involved in the calcification process, possibly by promoting apatite nucleation. Further research is needed to determine their exact function.Matrix vesicles were identified in the cartilage matrix of the upper tibial epiphyseal plate of normal mice. These vesicles were found throughout the plate and were not typically in contact with cartilage cells. They were concentrated in the longitudinal septa from the proliferative zone downward, varying in size (300 Å to 1 μm) and shape. The vesicles were bounded by unit membranes and contained materials of varying density, including rare ribosomes. A close association was found between matrix vesicles and calcification, with vesicles in juxtaposition to needle-like structures identified as hydroxyapatite and/or fluorapatite. Decalcification revealed similar structures in previously calcified cartilage. The study suggests that matrix vesicles may originate from cells and play a role in initiating calcification at the epiphysis. The vesicles were observed in various regions of the epiphyseal plate, with a characteristic distribution. Their fine structure varied, and they were not cytoplasmic projections from cartilage cells. The presence of matrix vesicles in the matrix of calcified cartilage and their association with apatite deposition suggest a role in calcification. Electron diffraction data supported the identification of hydroxyapatite and fluorapatite. The study concludes that matrix vesicles may be involved in the calcification process, possibly by promoting apatite nucleation. Further research is needed to determine their exact function.