Inclusion Criteria:

1. Histologically confirmed (by WHO classification) relapsed incurable non-Hodgkin
B-cell lymphoma of following subtypes; follicular grade I-IIIA, marginal zone, small
lymphocytic, lymphoplasmacytic, mantle cell.

2. Age ≥ 18 years

3. A pre-study WHO performance status of 0-1

4. Life expectancy should be ≥ 3 months

5. <25% tumour cells in bone marrow biopsy

6. CD37+, re-biopsy or test on existing tumour material if not known.

7. Measurable disease by radiological methods

8. Women of childbearing potential must:

1. understand that the study medication is expected to have teratogenic risk

2. have a negative pregnancy test

3. agree to use, and be able to comply with, effective contraception without
interruption, 4 weeks before starting study drug, throughout study drug therapy
and for 5 months after end of study drug therapy, even if she has amenorrhoea

9. Male subjects must agree to use condoms during intercourse throughout study drug
therapy and the following 5months

10. Patients previously treated with native rituximab are eligible

11. The patient is willing and able to comply with the protocol, and agrees to return to
the hospital for follow-up visits and examination

12. The patient has been fully informed about the study and has signed the informed
consent form

Exclusion Criteria:

1. Medical contraindications, including uncontrolled infection, severe cardiac,
pulmonary, neurologic, psychiatric or metabolic disease, steroid requiring
asthma/allergy, known HIV positive

2. Laboratory values within 15 days pre-registration:

1. Absolute Neutrophil Counts (ANC) ≤ 1.5 x 109 /l

2. Platelet count ≤ 150 x 109 /l

3. Total bilirubin ≥ 30 mmol/l

4. ALP and ALAT ≥ 4x normal level)

5. Creatinine ≥ 110 µmol/l (men), 90 µmol/l (women))

6. IgG ≤ 3 gr/l

3. Known CNS involvement of lymphoma

4. Previous total body irradiation, or irradiation of > 25% of the patient's bone marrow

5. Known history of HAMA

6. Chemotherapy or immunotherapy received within the last 4 weeks prior to start of
study treatment. Pretreatment with rituximab is allowed.

7. Pregnant or lactating women