Structural alterations in the spleen of eel Anguilla anguilla L., following exposure to a chemical spill in the Rhine h v e r in 1986, were investigated by means of light and electron microscopy. Relative abundance of reticulum and blood cells was quantified in comparison to controls. The data presented provide evidence of profound damage to the spleen and its cellular components. Histological and cytological modifications varied within a wide range from minor changes to cell death. Major cytological alterations of reticulum and blood cells included deviations in cytoplasmic density, loss of surface differentiations (cell junctions, pseudopodia), swelling of rnitochondria, formation of rnyelin-like membrane aggregations in the cytoplasm, mitochondria, lysosomes and nuclear membrane, as well as augmentation of lipid droplets. Severely damaged and necrotic cells were found either diffusely or focally distributed within the splenic reticulum. Melano-macrophages were strongly reduced in number and melano-macrophage centres were lacking. In contrast, considerable numbers of macrophages with intense lysosomal compartmentation, but free of pigments, were present. Spleens were further characterized by the absence of erythroblasts and plasma cells. Histological and cytological findings in the spleen of exposed eel indicate a severe impairment of the efficiency of the central splenic functions, in particular phagocytosis and cell-mediated immune response. Thus, chemical-induced patholopcal processes in the spleen have most likely been a contributing factor to the fatality of the chemical spill to the Rhine eel population. They are discussed in the context of earlier investigations on liver and intestine pathology of Rhine eel. Finally, cytological alterations of fish spleen are suggested as a potential biomarker for toxicant-induced damage in fish.