Skeletal myoblasts cloned from limb muscles of H-2Kb-tsA58 transgenic mice remained proliferative through at least 80 generations under conditions permissive for expression and function of the tsA58 gene product. When switched to nonpermissive conditions or implanted into muscles of nude mdx mice they underwent differentiation but, in one clonal cell line, a small proportion appeared to become quiescent muscle precursors in vivo. H-2Kb-tsA58 X mdx/mdx F1 male mice yielded dystrophin-deficient myoblasts. By such simple genetic crosses, H-2Kb-tsA58 transgenic mice provide a valuable tool for the rapid isolation of cell lines, myogenic or otherwise, bearing mutations of interest.