SummaryCon A administered intravenously in doses of 200-800 μg produced atrophy of lymphoid tissues, vascular endothelial damage and focal hepatic necrosis. At the highest dose given, up to 30% of the mice died within two days of what appeared to be acute hepatic failure. The injection of up to 2000 μg Con A ip produced only local inflammatory changes. The exact mechanism of Con A toxicity remains unclear.