A method is described for studying protein kinetics in pleural effusions in humans. In 15 patients with pleural effusions from a variety of causes, protein inflow was monitored by measuring the rate of appearance in the effusion of the plasma protein transferrin, radiolabelled in vivo by intravenous injection of 113InmCl3. Protein outflow was measured from the rate of appearance in the blood of intrapleurally administered 125I-albumin. Compartmental analysis has been used to determine rate constants k(in) and kout for the movement of labelled proteins into and out of the effusion, respectively. The mean value of k(in) for 15 patients was 9.4 x 10(-4) h-1 (range 2.2-21.4). The mean value of kout in the same patients was 28 x 10(-4) h-1 (range 6-68). Using appropriate assumptions where necessary, the absolute transfer rates of albumin and transferrin were also estimated. For albumin, the mean rate of outflow was 66 mg h-1 (range 27-158), compared to mean estimated inflow of 133 mg h-1 (range 36-381). Protein inflow was highest in a case of metastatic ovarian carcinoma with pleural and peritoneal seedlings, indicating high vascular permeability. Protein outflow was very low in a case of mesothelioma, suggesting severely impaired lymphatic drainage. The technique may prove to be a useful tool for studying mechanisms resulting in the formation of malignant and nonmalignant pleural effusions, and may also be useful for studying the effects of putative therapeutic intervention.