In 47 patients with pleural effusions of different etiology concentrations of the tissue polypeptide antigen (TPA) and tumour trypsin inhibitor (TATI) were measured. Malignant disease was found in 24 patients and benign one in 23 patients. In a group of patients with malignant pleural effusion the average concentration of TPA was 3102 U/L and of TATI 76.8 ug/L. In a group of patients with nonmalignant effusion these concentrations of TPA were 2381.8 U/L and of TATI 28.3 ug/L. Higher concentration of TPA was found in pleural effusion of malignant nature with specificity of 96% but with a small sensitivity of 12.5% and positive predictive value of 43%. TATI concentrations were also higher in malignant pleural effusion with good sensitivity and specificity (78%, 50% respectively) and positive predictive value of 66%.