Although the potential of retinal grafts to provide the host eye with rod cells is presently well established, the possibility of grafting cone photoreceptors has not been documented. In this study, the neural retinas of two Cebus apella monkey fetuses were xenografted into immunosuppressed Fischer 344 adult rats. Histological analysis showed intimate apposition between the grafted donor cells and the neighboring host rat retina. The transplanted cells survived well and often overgrew the boundaries of the host retina, expanding into the host vitreous cavity. These cells formed histogenetically differentiated structures predominantly populated by the photoreceptors. When transplanted into a foreign environment, donor cells formed inner segments which exhibited the basic morphology of cells developed in situ. This study demonstrates that embryonic monkey neural retina is a viable source of xenograft material. It also indicates that an advanced embryonic stage is not a deterrent to survival and differentiation of grafted primate neuroretinal cells. The successful transplantation of these cells, especially under the relatively adverse conditions of a xenograft, raises the hope that retinal transplantation may in fact be a useful technique for repairing both rod and cone function in damaged retinas of higher animals including humans.