The binding of eleven biotin- or peroxidase-coupled lectins with different carbohydrate specificities to tumour tissue and remaining morphologically normal retina was studied in ten formalin-fixed and paraffin-embedded human eyes with retinoblastoma. In undetached retinas, outer and inner segments of photoreceptors bound concanavalin A (ConA) as well as Lens culinaris (LCA), wheat germ (WGA) Ricinus communis (RCAI) and peanut (PNA) agglutinins. Both nuclear and plexiform layers bound ConA, LCA and, in some specimens, WGA and RCAI. These results agree with those obtained with normal adult human retina, the main difference being that PNA labelled some rods in addition to cones in the retinoblastoma eyes. Flexner-Wintersteiner rosettes reacted with ConA and LCA, and often with WGA, PNA and RCAI. Undifferentiated retino-blastoma cells always bound ConA and LCA, and in some tumours WGA, PNA and RCAI. Pretreatment with neuraminidase increased the number of cells that bound PNA and RCAI, but diminished binding of WGA. Pokeweed mitogen and Bandeiraea simplicifolia I, Dolichos biflorus, soybean, Ulex europaeus I and Lotus tetragonolobus agglutinins labelled only vascular endothelial cells. Retinoblastoma cells most closely resembled photoreceptor cells in their lectin-binding patterns.