- Various biochemical and physiochemical indices in the pleural fluid were determined in 20 patients with malignancy and pleural effusion. Significantly higher levels of lactic dehydrogenase (p < 0.05) and lower ratios of effusion/plasma (E/p) glucose (p < 0.01), were present in the 12 in whom malignant cells were found in pleural fluid. There were correlations between pH and E/P glucose (r = 0.91, p < 0.001). Nine of 12 pleural fluids from this group fulfilled at least one of three conditions: low pH (< 7.24), low E/P glucose (< 0.8) and low E/B bicarbonate (< 0.99). We suggest that low pH, low E/P glucose and low E/B bicarbonate values are reliable indicators of the presence of malignant cells in the pleural fluids of patients with neoplastic diseases.