NK cell CD16 binding to rituximab is blocked by complement fixation. U-bottomed 96-well microtiter plates were coated with varying concentrations of rituximab. Wells were incubated with media, 50% serum, or 50% heat-inactivated serum. After washing, NK cells were allowed to sit on plate for 30 minutes at room temperature. High absorbance resulted from NK-cell pelleting that occurred in the absence of binding to rituximab. All samples were run in triplicate (n = 3). Data are representative of 3 independent experiments. Error bars represent SD of the mean.