27 surgical specimens of retinoblastoma were transplanted into the anterior chamber of athymic nude mice (BALB/cA nu/nu), and 8 of them proliferated progressively filling the eyes. Among 7 specimens originally showing the rosette type, 1 case exhibited a typical Flexner-Wintersteiner rosette but the other 6 showed only an incomplete rosette after transplantation. One specimen originally showing the undifferentiated type exhibited the same cell type after transplantation. After the 2nd and 7th passage in the nude mouse, a chromosome analysis of the tumor cells confirmed that the cells were from the original human retinoblastoma. After the 7th passage the effects of radiation on the ultrastructure of the transplanted retinoblastoma cells were studies. The Linac β-ray was radiated at single doses from 300 to 1500 rad, and the eyes were fixed on the 11th day and examined by light and electron microscopy. With 300 and 450 rad, no apparent change was found, but over 600 rad, degenerating cells being polygonal in shape and stained heavily with toluidine blue, were seen; they increased in number by increasing the dose of radiation. These degenerating cells contained vacuoles, dilated endoplasmic reticulum, myelinated figures, laminated and dense particles and empty mitochondria. They also showed partial disappearance of the cell membrane and karyorrhexis. By increasing the dose of radiation, the mitotic figures decreased, but even with 1500 rad, a few mitotic figures were still encountered.