L6 myoblasts were initially isolated by Yaffe from the first two passages of primary cultures of rat thigh muscle in the presence of methylcholanthrene. L6 cells can fuse in culture medium to form multinucleated myotubes and striated muscle fibers. The degree of fusion of L6 cells decreases with increasing passage number; therefore, L6 cells should be cryopreserved at low passage and periodically recloned to select for cells with strong fusion capability. If the cells in the culture vessel are allowed to reach confluence, the myogenic component of L6 cells will rapidly deplete.