Provided is a method of treating an autoimmune disease in an animal comprising administering to the animal an antibody-DT mutant immunotoxin which routes by the anti-CD3 pathway, or derivatives thereof, under conditions such that the autoimmune disease is treated. In a further embodiment, the invention provides a method of treating T cell leukemias or lymphomas in an animal comprising administering to the animal an antibody-DT mutant immunotoxin which routes by the anti-CD3 pathway, or derivatives thereof, under conditions such that the T cell leukemias or lymphomas are treated.