Main Inclusion Criteria:

• Subjects with CD20-positive, B cell NHL who, after at least 2 prior therapies of probable clinical benefit, have relapsed or refractory disease.

The following histologies may be included*: lymphoplasmacytic lymphoma (formerly known as lymphoplasmacytoid lymphoma), splenic marginal zone B cell lymphoma, extranodal marginal zone B cell lymphoma of mucosa-associated lymphoid tissue, nodal marginal zone B cell lymphoma, follicular lymphoma, mantle cell lymphoma, diffuse large-cell B cell lymphoma, and mediastinal large B cell lymphoma. Small lymphocytic lymphoma will be included if it is a primary diagnosis and if the lymphoma cells are < 5.0 x 109/L (5000/mm3) in the peripheral blood. *Subjects enrolled in the preliminary efficacy cohorts must have relapsed, refractory, or persistent follicular lymphoma (persistent disease defined as computed tomography (CT) positive for 3 months after last treatment), and must not have received anti-CD20 targeted therapy within 3 months of receiving the first dose of test article. Subjects may be considered eligible after a single therapy of probable clinical benefit, if no further standard effective treatment is available in the opinion of the investigator. Prior CD20 immunophenotyping of tumors to document B cell NHL is acceptable. If such prior documentation is not available, then the immunophenotype of the current disease must be documented by fine-needle aspirate or biopsy, or by circulating CD20-positive NHL cells from peripheral blood before administration of test article.

• At least 1 measurable lesion that is 1.5 cm in at least 1 dimension by CT or magnetic resonance imaging (MRI), in an area of no prior radiation therapy, or documented progression in an area that was previously irradiated.
• Recovery to baseline or grade 1 [according to the National Cancer Institute (NCI) Common Terminology Criteria for Adverse Events (CTCAE), version 3.0] from all acute adverse effects of prior therapies (excluding alopecia).

Main Exclusion Criteria:

• Candidate for potentially curative therapy that is available to the subject, in the clinical opinion of the investigator.
• Diagnosis of chronic lymphocytic leukemia, Burkitt's lymphoma, primary effusion lymphoma, and/or precursor B cell lymphoblastic lymphoma.
• Prior treatments: radioimmunotherapy; allogeneic hematopoietic stem cell transplant (within 6 months of first dose of study drug); chemotherapy, cancer immunosuppressive therapy, growth factors (except erythropoietin), or investigational agents (within 4 weeks of first dose of study drug); major surgery not related to debulking surgical procedures (within 3 weeks of first dose of study drug).