The eye lens is made up ot two cell types, epithelial cells which cover the anterior face of the lens and fibre cells which make up the bulk of the lens. Fibre cells are derived from the differentiation of epithelial cell at the lens equator and this process continues throughout the life of an individual. No cells are ever lost from the lens, therefore cell survival factors are likely to be fundamental to the fibre cells. In addition recent evidence suggests that cataract can arise from the apoptotic loss of cells in the anterior epithelium of the lens. Evidence from other studies (see poster by Dewar et al.) suggest that proenkephalin can act as both a cell survival molecule and as a signal for programmed cell death. We have therefore used immunolocalisation methods to determine the distribution of proenkephalin in the eye lens from several species including human. Proenkephalin can be demonstrated by immunoblotting throughout the lens, ie. at all stages of fibre cell differentiation. However, immunolocalisation demonstrates changes in the cellular compartment in which proenkephalin is revealed during differentiation. Several different patterns can be demonstrated using monoclonal antibodies which recognise different non-enkephalin epitopes on proenkephalin; selective epitope unmasking may reflect differential association of proenkephalin with other proteins which is differentiation stage-specific. Plasma membrane staining occurs at the point in differentiation when the lens fibre cells lose their nuclei. The pattern of nuclear staining colocalises with that of p53. These findings will be discussed in the light of other evidence (see Dewar et al.) that proenkephalin can induce apoptosis in a p53 dependent manner in other cell types including cultured lens cells.