Rituximab Plus CAMPATH in Patients With Relapsed/Refractory Low-Grade or Follicular, CD20-positive, B-cell NHL

Inclusion Criteria: * For the Phase I portion of the study, patients must have pathologically confirmed diagnosis of low-grade or follicular, CD20-positive, B-cell, non-Hodgkin's lymphoma that has relapsed or is refractory. For the Phase II portion of the study, patients must have a pathologically confirmed diagnosis of low-grade or follicular, CD20-positive, B-cell, non-Hodgkin's lymphoma (Follicular, predominantly small cleaved or follicular, mixed small cleaved and large cell, International Working Formulation classification B or C or REAL classification follicular center grade 1,2) that has relapsed or is refractory. * Previously treated with at least one anti-cancer regimen for NHL * Measurable disease (lesions that can be accurately measured in 2 dimensions by CT scan with a greatest transverse diameter of \>/= to 2cm or palpable lesions with both diameters of 2cm or more) * Life expectancy of at least 12 weeks * WHO performance status or 0 or 1 * Adequate marrow and organ function (as defined in the protocol) * Completed major surgery, radiotherapy, chemotherapy, immunotherapy or biotherapy/targeted therapies at least 4 weeks prior to study entry (6 weeks if treated with a nitrosourea or mitomycin). Patients must have recovered from all prior treatment toxicity to Grade 1 or less, exclusive of alopecia. Exclusion Criteria: * Prior combination therapy with rituximab and CAMPATH; prior therapy with either agent alone is permitted * A history of a T-cell lymphoma * Known AIDS-related HIV-positive lymphoma * For the Phase II portion of the study (once MTD has been determined), bulky disease, ie, any single mass \>10cm or circulating malignant cells of 25,000/uL or more * Prior autologous bone marrow or stem cell transplant within 6 months of study entry * Prior allogeneic bone marrow transplant or organ transplant * Prior radiotherapy to the only site of measurable disease * Medical condition requiring chronic use of oral, high-dose corticosteroids * Use of investigational agents within 30 days of study enrollment * Past history of anaphylaxis following exposure to humanized monoclonal antibodies * Known, active, infection, including HIV positive * Diagnosis of another malignancy within the previous five (5) years, unless the probability of recurrence of the prior malignancy is \< 5%. Patients with curatively treated early stage squamous cell carcinoma of the skin, basal cell carcinoma of the skin, cervical intraepithelial neoplasia (CIN), and patients with a history of malignant tumor in the past that have been disease-free for at least 5 years * Active central nervous system (CNS) involvement with lymphoma * Pregnant or nursing women * Any significant concurrent disease or illness that would, in the opinion of the investigator, compromise patient safety or compliance, or interfere with the interpretation of study results * Active hepatitis or a history of prior viral hepatitis B or C, or positive hepatitis B serologies without prior immunization