Human embryonic retinas (postconceptional age 3-10 weeks) with or without retinal pigment epithelium were grafted to the retina of immuno-suppressed adult rat hosts. The development of the xenografts was followed up to 37 weeks of total age by histology and by immunohistochemistry for S-antigen. The donor tissue became rearranged in folded sheets with rosettes. The grafts developed approximately according to their intrinsic timetable, but with a developmental delay in the later stages. Occasionally, the grafts were well fused with the host retina. At 13 weeks of total age, the grafts contained areas of inner plexiform layer with presumptive ganglion cells, one neuroblastic layer, and cone precursor cells around rosettes. At 19 weeks, an outer plexiform layer and inner segments of the cones started to form. At 20 weeks, the first immunoreactivity for S-antigen was observed in photoreceptor precursors. Cone inner segments were clearly distinguishable at 28 weeks, and more S-antigen-positive rods were seen. At 31 weeks, rods were more differentiated, showing S-antigen-positive inner and outer segments. An inner limiting membrane with an apparent ganglion cell layer was only seen in one cograft of retina and retinal pigment epithelium at 37 weeks, indicating an important role of retinal pigment epithelium for graft differentiation. This study shows that human embryonic retina can be grafted to immuno-suppressed adult rat retina with long-term survival. A high degree of maturation can be obtained in the grafted tissue comparable to the layering of newborn human retina. It appears that most cell types develop. This model opens up possibilities for studying human retinal development with the goal of reaching a treatment for human degenerative retinal disorders.