Concentrations of carcinoembryonic antigen (CEA) and carborhydrate antigen (CA) 50 were measured in pleural effusion and sera of 57 patients with bronchogenic carcinoma and in 73 patients in whom the effusion was the sequela of tuberculous pleurisy. In the group with bronchogenic carcinomas, planocellular was confirmed in 19, microcellular in 17, macrocellular in 2, and adenocarcinoma in 18, while in 1 patient it was not possible to determine the histopathologic structure. The diagnosis of pleural disease was established upon the cytologic examination of the effusion and histopathologic examination of the pleural sample obtained by blind percutaneous needle biopsy or following pleuroscopy. CEA concentration in the sera of patients with bronchogenic carcinoma was significantly higher than in the patients with tuberculosis (p < 0.001), with sensitivity of 44% and ideal specificity and positive predictive value of 100%. In the same group highly significant difference of mean values of CEA concentrations in pleural effusion (p < 0.001), was also found with sensitivity of 60%, significant specificity of 99% and positive predictive value of 97%. CA 50 concentrations in the sera of patients with lung carcinoma were significantly higher than those in the sera of patients with tuberculous pleurisy (p < 0.05), and the sensitivity was 50%, while the specificity was 94% and positive predictive value was 75%. Significantly higher was also the value in the pleural effusion (p < 0.05), but the sensitivity was slightly lower--40%, but specificity was favorable as well as the positive predictive value (94 and 86%, respectively). The results indicate the significance of the determination of CEA and CA 50 in the sera and pleural effusion in the differentiation of malignant from tuberculous pleural effusion.