We submitted 83 consecutive patients with pleural effusion to routine clinical investigation; 57 were diagnosed as malignant, 18 as benign, and 8 were not diagnosed. Pleural fluid and serum were analysed for carcinoembryonic antigen (CEA), acid glycoprotein (AGP), antichymotrypsin (ACT), C-reactive protein (CRP), alpha 2-pregnancy associated glycoprotein (alpha 2-PAG) and ferritin. Multivariate discriminant analysis was performed on the results of the protein measurements. CEA and ACT values in serum and fluid were found to give a good discriminating function between the benign and malignant groups. The use of such an analysis, in a clinical context, is discussed.