A transplantable intraocular retinoblastoma-like tumour growing in F 344 rat eyes and tissue cultures suitable for therapeutic experiments is characterized. The tumours grew in the vitreous and infiltrated the internal layers of the retina. Feeder vessels from the retina supplied the tumours. Few and often incomplete rosettes were seen, and calcification was sparse in areas of necrosis. Stainings for glial fibrillary acidic protein, S-100 protein, neuron-specific enolase and desmin were negative. Ultrastructural analysis brought out a close correspondence between the tumour cell and the human retinoblastoma and other virus-induced retinoblastoma-like tumours. The experimental tumour cells lacked cilia, but had neurosecretory granules. Chromosome analysis showed a modal chromosome number in the triploid range, and 6 marker chromosomes were demonstrated. Flow-cytometric analysis showed a S-phase cell fraction of 42%, and chromosome stability was suggested by a tumour-cell DNA index which remained stable for more than 3 months.