Uninvolved retina in the posterior pole in an enucleated human eye was examined by EM for retinoblastoma. A mitotic figure was occasionally found in the outer and inner nuclear layers. The dividing cells were separated from the tumor mass by 450-600 .mu.m. They were identified as immature Mueller cells based on fine structural features. The existence of dividing cells in the posterior retina after birth was abnormal and indicated immaturity of retinal development.