Human retinal epithelial cells dividing in culture have been radiolabelled with tritiated thymidine and subsequently transplanted and identified on Bruch's membrane of owl monkey by autoradiography. Such cells have been followed from 2 hours to 7 days after transplantation. The transplant cells reattach within 2 hours to the basement membrane left on Bruch's membrane after the host epithelial cells have been removed by suction after trypsinization. Within 6 to 24 hours they form a layer on Bruch's membrane with junctional complexes between cells and an apical-basal polarity characteristic of such cells in culture or after retinal detachment. After 2 to 7 days multilayers form by continued mitosis but no transplant cells invade the choroid as long as Bruch membrane is intact. Within this time macrophages begin to appear in the choriocapillaris under the transplant. These cells migrate through Bruch's membrane and between the transplant cells but have not been seen phagocytizing transplant cells. Reattachment of the neural retina to the transplant layer has not been attempted but seems technically possible.