Inhibitory effect of transudative pleural fluid and nonmalignant ascites is dose dependent and abrogated by heat inactivation. PBMCs and Raji cells were mixed at a 1:1 ratio for 20 hours in various concentrations of pleural fluid, ascites, or heat-inactivated samples in the presence of 5 μg/mL rituximab. NK cell–surface marker expression was determined by using flow cytometry with gating on CD3⁻, CD56⁺ lymphocytes. (A) NK cell CD16, expressed as median fluorescence, after incubation with various concentrations of pleural fluid, ascites, or heat-inactivated samples (n = 3 samples per group). (B) NK cell CD54, expressed as a percentage of CD54 bright, after incubation with various concentrations of pleural fluid, ascites, or heat-inactivated samples (n = 3 samples per group). Error bars represent SD of the mean.