Complement-dependent cytotoxicity of CD20 antibodies. Daudi cells (0.1 × 10⁶) were incubated with CD20 antibodies rituximab, HuMab 7D8, HuMab 7D8-K322A or tositumomab (10 μg/mL) at room temperature for 15 minutes. Normal human serum (final concentration 20%) was added as a source of complement, and cells were incubated for 45 minutes at 37°C. Propidium iodide was added, and cells were analyzed by flow cytometry. Results are shown as percentage of propidium iodide–positive cells proportional to total cell number (% lysis) and are representative of 3 separate experiments.