This is a Phase I/II clinical trial aimed at treating patients with T-cell lymphomas (T-cell blood cancers). The drug consists of a toxin, called diphtheria toxin, which is attached to an antibody that can specifically target cancerous T-cells. As this is a Phase I/II trial, it has not been tested in humans. Our primary objectives are, therefore, to determine how much of the drug can be given to an individual and what are the toxicities associated with the drug. It is anticipated that approximately 40 patients will be enrolled in this study over two years. Patients will receive full supportive care during the course of the study. Participation in the study will require a 5 day stay in the hospital, as well as frequent (2-3 times/week) outpatient labs (blood draws) for the first 30 days. Patients with partial or complete remissions at their 1 month follow up visit will have another follow-up visit on day 60, then every three months for 1 year, followed by annual visits to assess duration of the response.