Technical Abstract: To characterize and compare the toxicity and pathology of glycosidase inhibitors swainsonine, castanospermine, calystegine A3, calystegine B2 and calystegine C1, 48 Swiss Webster, male mice were randomly divided into 16 groups of 3 animals each. All mice were implanted with subcutaneous osmotic minipumps that delivered the different groups with the following target doses: swainsonine- 0.1, 1.0 and 10.0 mg/kg/day; castanospermine- 1.0, 10.0 and 100.0 mg/kg/day; calystegine A3- 1.0, 10.0 and 100.0 mg/kg/day; calystegine B2- 1.0, 10.0 and 100.0 mg/kg/day; calystegine C1- 1.0, 10.0 and 100.0 mg/kg/day; and the control- equal saline volumes. After 28 days of dosing all the mice were euthanized, perfused with Karnowsky’s fixative, necropsied and examined using light and electron microscopy. All groups were unchanged clinically with the exception of the high dose swainsonine - treated mice that gained less weight, ate less, and became more nervousness than controls. Histologically, the high dose swainsonine treated animals developed severe vacuolar degeneration of the renal tubular epithelium, thyroid follicular cells, and reticuloendothelial cells of the lymph nodes, spleen, lung, liver and thymus. They also developed mild vacuolation of neurons, ependyma, adrenal cortex, myocardial epicytes and interstitial cells and gastric parietal cells. Low dose swainsonine treated mice had minimal vacuolation of the thyroid epithelium and renal tubular epithelium. Mice dosed with castanospermine and the various calystegines were clinically normal; however, the high dose castanospermine treated mice developed mild vacuolation of the renal tubular epithelium, the thyroid follicular epithelium, hepatocytes and skeletal myocytes. Special stains and lectin histochemistry showed that swainsonine and castanospermine induced vacuoles contained mannose-rich oligosaccharides. Castanospermine-induced vacuoles also contained glycogen. High dose calystegine A3 treated mice had increased numbers of vacuolated cells in the hepatic sinusoids. Electron microscopy, lectin histochemistry and immunohistochemistry suggest that these are vacuolated endothelial cells that contain glycoproteins or oligosaccharides with abundant terminal N-acetylglucosamine residues. No additional histologic lesions were identified in any of the calystegine treated mice. These findings indicate that: 1) swainsonine produced clinical and histologic lesions similar to locoweed poisoning; 2) castanospermine caused vacuolar changes in the kidney and thyroid gland with minimal glycogen changes in hepatocytes and myocytes; and 3) other than calystegine A3 which produced minimal hepatic change, the calystegines did not induce significant clinical or histologic lesions. These findings suggest that under these conditions calystegines and castanospermine are much less toxic than swainsonine. This suggests that most of the toxicity of plants containing mixtures of these glycosides is most likely due to swainsonine.