This is a single-arm, single institution, Phase II study to test the use of low-dose methotrexate in combination with I-131 tositumomab for its ability to lower the rate of (human anti-mouse antibody) HAMA formation in patients with previously untreated low-grade follicular lymphoma. Low-dose methotrexate will be given beginning 3 weeks prior to the first infusion of I-131 tositumomab (4 weekly doses) and continued for 6 weeks (10 total doses), the period of time during which the development of HAMA is most detrimental. A total of 61 patients will be enrolled. The primary endpoint of the study is the determination of the rate of HAMA conversion within the first seven weeks following treatment. The secondary endpoints include response rates, progression-free and overall survival, and safety.