This study will investigate the efficacy of weekly intravenous obinutuzumab [GA101 (RO5072759)] monotherapy, in patients with relapsed CD20+ indolent Non-Hodgkin's Lymphoma. Patients will be randomized to receive either GA101 or rituximab, given as four weekly infusions. At the conclusion of the initial trial patients may be eligible to continue therapy up to 24 months. The anticipated time on study treatment is 3- 24 months, and the target sample size is 100-500 individuals.