C5-depleted serum inhibits rituximab-mediated ADCC. Raji cells were labeled with ⁵¹Cr and incubated with purified NK cells in various E/T ratios, 5 μg/mL rituximab, and either C5-depleted serum, heat-inactivated C5-depleted serum, or media. Percentage of specific lysis was measured based on ⁵¹Cr release. All samples were run in triplicate (n = 3). Data are representative of 3 independent experiments. Error bars represent SD of the mean.