Transudative pleural fluid and nonmalignant ascites contain active complement. ELISA analysis showing C1q is detectable in transudative pleural and ascitic fluid but at levels lower than that seen in serum (A). Raji cells were incubated with 5 μg/mL rituximab and various concentrations of serum, heat-inactivated serum, ascites, or pleural fluid, and the percentage of cells showing deposition of C3b was determined by flow cytometry. Deposition of C3b was seen on the cell surface in all samples except heat-inactivated serum (B). The percentage of viable target cells remaining after the addition of pleural fluid (C) or ascites (D) was determined with the use of flow cytometry by counting remaining annexin V– and propidium iodide–negative target cells (n = 3). Error bars represent SD of the mean.