To study proliferation of the retinal pigment epithelium (RPE) after experimental retinal detachment, an antibody that recognizes the specific proliferating cell nuclear antigen (PCNA) in proliferating cells was used. Retinal detachment was produced in cats by subretinal injection of 0 25% solution of Healon through a micropipette. At different intervals the animals were sacrificed and eye globes were fixed in 10% at malin and embeded in paraffin. Histologic sections were processed for immunohistochemisty using an antibody to detect the PCNA protein. Labeled RPE cells were identified, and the proliferative response was quantified. In the retina detached at 0 5, 1, 2, 3, 4, 5, 6, 7, 10, 14, 20, 30, and 60 days, the number of PCNA labled cells was 0 055, 0 444, 0 861, 1 972, 3 139, 5 833, 6 028, 4 917, 3 333, 2 195, 1 083, 0 195, and 0 056 cells per millimeter of retina, respectively. In non detached retina of the same eye, approximately from 0 to 0 639 cells per millimeter of retina were labeled. The difference of PCNA labeled cells between detached and non detached retina of the same eye was significant. These results indicate that RPE cells are induced to proliferate when they lose contact with neural retina.