The photoreceptor cells and retinal pigment epithelium (RPE) represent a highly differentiated functional unit that may be disrupted in a variety of retinal diseases. We studied the localization and distribution of carbohydrate residues in normal and pathologically altered human retinas using lectin histochemical techniques. Paraffin-embedded tissue sections were investigated for binding sites with six lectins: concanavalin A (Con A), wheat germ agglutinin (WGA), Ricinus communis agglutinin (RCA I), peanut agglutinin (PNA), soybean agglutinin (SBA), and Ulex europaeus agglutinin (UEA I). Lectin binding to both the photoreceptor segments and the RPE was observed in the normal retina with Con A, WGA, RCA I, and PNA, particularly to the distal outer segments and to the apical portion of RPE cells. Disturbance of the photoreceptor-RPE cell complex occurred in eyes with damaged outer retinal layers and those with retinal detachment. Photoreceptor cells with a varying degree of inner and outer segment degeneration revealed an overall decrease in lectin-binding sites as well as alterations in their cellular topography. Reactive and proliferating RPE, however, showed a markedly increased reaction intensity for Con A, WGA, RCA I, and PNA as well as additional binding sites for SBA in macrophage-like variants. The data are taken as evidence that in the normal retina, a characteristic location and distribution of lectin-binding sites as well as correspondences in certain cellular glycoproteins of both photoreceptor cells and the RPE are indicative of structural and functional integrity. Disturbance of the photoreceptor-RPE cell complex is accompanied by changes in the expression of lectin-binding sites on both cell types.