Although methods for measuring colloid clearance rates have been described, they are not commonly used. Rather, most clinicians rely on the relative radiocolloid accummulation in the liver and spleen as estimated by visual inspection of liver scans. This method lacks objectivity, however, and only indirectly reflects the rate of radiocolloid clearance. We have developed a noninvasive kinetic technique for measuring radiocolloid clearance by the liver, spleen, and other reticuloendothelial tissues. The clerance-rate constants obtained by this technique appear to differentiate among cirrhosis, fatty metamorphosis, hepatitis, and normal function. In normal subjects, the mean clearance-rate constants for the liver, spleen, and extrahepatosplenic reticuloendothelial system were 16.0, 1.4, and 3.4 ml/min per 100 ml of plasma, respectively. The mean hepatic clearance-rate constant was normal in hepatitis (16.8 ml/min per 100 ml), reduced in cirrhosis (5.7), and slightly reduced in fatty metamorphosis (10.4). Both the hepatic-to-splenic and the hepatic-to-extrahepatosplenic ratios of clearance-rate constants were reduced in cirrhosis and slightly reduced in fatty metamorphosis and hepatitis. Interestingly, the splenic clearance-rate constants were normal in these three diseases.