The diagnosis of feline alpha-mannosidosis is made by demonstrating deficient activity of the enzyme alpha-mannosidase or an elevation of its undergraded substrate in body fluids or tissue. In this study the storage of specific sugar residues in the brain and spinal cord in a case of feline alpha-mannosidosis was examined by means of nine different biotinylated lectins and the avidin-biotin-peroxidase method on formalin-fixed, paraffin-embedded tissue sections. The lectin staining pattern strongly correlated with the known biochemical findings of the stored oligosaccharides in feline alpha-mannosidosis and was different from the lectin reactivity of normal cat tissues. This confirms that lectin histochemistry is a simple, relatively inexpensive and reliable method for diagnosing alpha-mannosidosis in cats.