The retinoblastoma gene (RB1) has been implicated as a tumor suppressor in multiple myeloma (MM), yet its role remains unclear because in the majority of cases with 13q14 deletions, un-mutated RB1 remains expressed from the retained allele. To explore the role of Rb1 in MM, we examined the functional consequences of single- and double-copy Rb1 loss in germinal center B cells, the cells of origin of MM. We generated mice without Rb1 function in germinal center B cells by crossing Rb1Flox/Flox with C-γ-1-Cre (Cγ1) mice expressing the Cre recombinase in class-switched B cells in a p107-/- background to prevent p107 from compensating for Rb1 loss (Cγ1-Rb1F/F-p107-/-). All mice developed normally, but B cells with two copies of Rb1 deleted (Cγ1-Rb1F/F-p107-/-) exhibited increased proliferation and cell death compared with Cγ1-Rb1+/+-p107-/- controls ex vivo. In vivo, Cγ1-Rb1F/F-p107-/- mice had a lower percentage of splenic B220+ cells and reduced numbers of bone marrow antigen-specific secreting cells compared with control mice. Our data indicate that Rb1 loss induces both cell proliferation and death in germinal center B cells. Because no B-cell malignancies developed after 1 year of observation, our data also suggest that Rb1 loss is not sufficient to transform post-germinal center B cells and that additional, specific mutations are likely required to cooperate with Rb1 loss to induce malignant transformation.