Inclusion Criteria:

• Histologically confirmed CD19-positive non Hodgkin's lymphoma.
• Relapsed or refractory after standard treatments and with no curative option with conventional therapy.
• Bidimensionally measurable disease.
• No evidence of cerebral or meningeal involvement by lymphoma.
• Age >/= 18 years.
• ECOG performance status 0 to 2.
• Life expectancy of at least 3 months.
• Signed informed consent form prior to enrollment.

Exclusion Criteria:

• Burkitt's lymphoma, Lymphoblastic lymphoma, Chronic lymphocytic leukemia (Small lymphocytic lymphoma may be included).
• Chemotherapy or radiation therapy or other investigational agents within 4 weeks prior to entering the study.
• Previous radioimmunotherapy within 12 weeks.
• Patient with active infections or HIV infection.
• Known active viral hepatitis.
• Known intolerance to infused protein products or maytansinoids (an anti-tumor medication that is a derivative of a natural plant).
• Any serious active disease or co-morbid condition, which in the opinion of the principle investigator, will interfere with the safety or with compliance with the study.
• Poor bone marrow reserve as defined by absolute neutrophil count < 1.5 x 10^9/L or platelets < 100 x 10^9/L.
• Baseline QTc interval > 500 msec (determined as the median of three EKGs done within a 30-min period) (Grade >/= 3 NCI-CTC v3.0)
• Poor organ function as defined by one of the following: a.) Total bilirubin > 1.5 x ULN (>3 x ULN for patients with liver involvement); b.) AST, ALT > 2.5 x ULN (> 5 x ULN for patients with liver involvement); c.) Serum creatinine > 2 x ULN and/or creatinine clearance < 60 mL/min
• Pregnant or breast-feeding women (sexually active females of childbearing potential must have a negative serum or urine pregnancy test).
• Refused to use effective birth control methods if applicable. Women that can have children must use a medically acceptable method of birth control while participating in the study (birth control pill, intrauterine device or injection of Depo-Provera) and for at least 30 days after the last dose of study medication.