Diagnosis and disease status:

• Subjects with CD20+, B-cell, Non-Hodgkin's lymphoma of low-grade or follicular histology with measurable relapsed or unresponsive disease after prior therapy; mantle cell and chronic lymphocytic leukemia subtypes are excluded.
• Subjects who previously received a single-agent course of rituximab and showed no tumor response, or had a response lasting < 6 months. The previously administered rituximab must have included at least 75% of the standard 4-week regimen (4 x 375 mg/m2). A record of the previous rituximab treatment and response must be available as a source document at the site.

Exclusion:

• Subjects who showed no tumor response or a response lasting <6 months to treatment with Rituximab in combination with Chemotherapy or another therapeutic modality (radiation or radioimmunoconjugates).
• HIV positive.
• Symptomatic thyroid disease requiring medical intervention other than replacement treatment for hypothyroidism.
• Clinically significant cardiac, pulmonary, and /or hepatic dysfunction (if subject has history of congestive heart failure or myocardial infarction, must have been stable for at least 6 months, and have no current symptoms
• If cardiac ejection fraction has been measured, it must be greater than 50%.