Background: Cynomolgus monkeys are widely used animal models in biomedical research. The differences between cynomolgus monkey and human B cells are not completely understood. However, these differences are of crucial importance for interpretation of data from studies on new therapeutic agents aimed at B-cell depletion, such as anti-CD20 monoclonal antibodies. Methods: Multicolor fluorescence-activated cell sorting analysis of peripheral blood B cells was performed on samples treated ex vivo with the anti-CD20 therapeutic monoclonal antibody, Rituxan, in a whole blood matrix. Results: In contrast to humans, cynomolgus monkeys had two distinct B-cell subsets, CD20 high CD40 low CD21 Ϫ and CD20 low CD40 high CD21 ϩ . These B-cell subsets had a 2.5-fold difference in the EC 50 for Rituxan binding and differed significantly in their in vitro susceptibility to Rituxan depletion. Human B cells were similar to the