The mechanism of rituximab-mediated depletion of nonmalignant CD20+B cells remains to be clarified. In this report, we examine contributions of complement- and cell-dependent killing to the rituximab-mediated depletion of cynomolgus monkey B cells in the in vitro assay. B cell depletion was assessed in whole blood, buffer, autologus plasma (plasma), heat-inactivated plasma (H/I plasma), and cobra venom factor (CVF)-treated plasma matrices in cynomolgus monkey and human samples. Rituximab-mediated B cell depletion in buffer appeared to be greater than that in whole blood or in autologus plasma. Heat inactivation of plasma resulted in the degree of B cell depletion closer to that seen in buffer, whereas CVF treatment of plasma had no effect on B cell depletion. Addition of IgG to the buffer decreased the degree of B cell depletion. The results of these studies imply that (i) plasma components (including complement) are not the mediators of the rituximab-triggered B cell depletion in the in vitro assay, suggesting that cell-mediated mechanisms are likely to be responsible for in vitro killing of normal B cells, and that (ii) some plasma components appear to inhibit rituximab-mediated B cell depletion in the in vitro assay, with IgG identified as a possible inhibitor component.