Study of the ultrastructure of regenerating synovial membrane of rabbits at intervals ranging from two to 100 days after subtotal synovectomy of the knee, using the electron microscope, revealed that macrophages became Type A cells, fibroblasts became Type B cells. The new membrane is established by thirty days and continues to mature until 100 days at which time regenerated synovium cannot be distinguished from the normal under the electron microscope.