Fresh surgical specimens of human retinoblastoma were successfully transplanted into the anterior chamber of the nude mouse (a homozygous mutant, nu/nu, with a severe defect in cellular immunity), filling the eyes but failing to grow subcutaneously. Retinoblastoma cells from an established cell line, Y-79, spread from the intraocular injection site to invade the orbit, optic nerve and brain and formed large tumors when implanted s.c. Tumor cells injected into the anterior chamber of immunologically normal littermates (heterozygotes, nu/+) survived for varying periods in the anterior chamber but showed little growth.