A Trial of the Safety of Escalating Doses of PRO131921 in Patients With Relapsed or Refractory Indolent Non-Hodgkin's Lymphoma

* Signed informed consent * Ability and willingness to comply with the requirements of the study protocol * Age ≥ 18 years * Documented history of histologically confirmed CD20-positive follicular NHL (Phases I and II); for Phase I, other CD20-positive indolent lymphomas (marginal zone lymphoma and small lymphocytic lymphoma) are also included. Patients with a history of a CD20-positive NHL of mixed (indolent/aggressive) histology may also be included in Phase I if the most recent relapse biopsy shows only indolent NHL * Indolent lymphoma (Phase I) or follicular lymphoma (Phase II) that either has relapsed after a documented history of response (CR, CRu, or PR) of ≥6 months to a rituximab-containing regimen or was refractory to a previous rituximab-containing regimen * Bi-dimensionally measurable disease, with at least one lesion \>1.5 cm in its largest dimension * Absolute B-cell count ≥LLN at screening (Phase I only) * ECOG performance status of 0, 1, or 2 * For patients of reproductive potential (males and females), use of a reliable means of contraception during the study and for 1 year following the last dose of study treatment * For females of childbearing potential, a negative serum pregnancy test within 14 days prior to enrollment Exclusion Criteria: * Prior use of anti-CD20 monoclonal antibody therapy (other than rituximab or radioimmunotherapy) * Prior use of any non-CD20-targeted monoclonal antibody therapy within 6 months of enrollment * Current or recent lymphoma treatment * History of severe allergic or anaphylactic reactions to human, humanized, chimeric, or murine monoclonal antibodies * Evidence of significant uncontrolled concomitant diseases, such as cardiovascular disease, nervous system, pulmonary, renal, hepatic, endocrine, or gastrointestinal disorders * Evidence of myelodysplasia or myelodysplastic changes on bone marrow examination * Untreated or persistent/recurrent malignancy (other than lymphoma) * Known active bacterial, viral, fungal, mycobacterial, or other infection * A major episode of infection requiring hospitalization or treatment with IV antimicrobials within 4 weeks of screening or oral antimicrobials within 2 weeks of screening * History of recurrent significant infection or bacterial infections * Positive hepatitis B or C serology * Positive human immunodeficiency virus (HIV) serology * Pregnancy or lactation * Central nervous system lymphoma * Recent major surgery within 4 weeks of screening, other than diagnostic surgery