The anti-CD20 monoclonal antibody, rituximab, and interferon alpha (IFN) are active agents in advanced stage, indolent lymphoma. Some data obtained in vitro suggest upregulation of CD20 by IFN, and clinical trials have reported additive or synergistic activity of IFN with rituximab. A prospective phase II study in advanced stage, follicular and non-follicular indolent lymphoma was performed. Peginterferon alpha 2b (pegIFN) 0.5 microg/kg s.c. weekly x 6 and rituximab 375 mg/m2 i.v. weekly x 4 (beginning on week 3 of pegIFN) was administered. Quantitative CD20 antigen was measured pre-treatment and pre-rituximab in lymph node aspirates. Nine patients were treated: one complete response and three partial responses; however, all relapsed within 12 months and two were withdrawn for grade 3 toxicity (both with serum sickness). No up-regulation of CD20 was documented in seven patients studied (median change in CD20: decrease by 11.9%). PegIFN plus rituximab as delivered in this study is not recommended. PegIFN does not appear to upregulate CD20 expression in peripheral lymph node tumor cells.