Tumor formation in nude mice (nu/nu Balb c outbred) inoculated with cells from four new permanent human glioma cell lines was studied. Three of these lines had previously been shown to display features of striated muscle in vitro. Histochemical and immunochemical techniques together with electron microscopic study confirmed that striated muscle differentiation continued to be expressed in vivo. Two of the cell lines arguably showed greater striated muscle differentiation in vivo, whereas one has lost this ability. In one of the two, further mesodermal differentiation was evident with the formation of cartilage.