The mechanism of (103)Ru-uptake in tumors was investigated through the incubation of rat ascites hepatoma cells (AH-130) in vitro with various concentrations of Ru-chloride containing (103)Ru-chloride as a tracer. Quantitative analysis of Ru binding to the cells indicated that ascites hepatoma cells contained high and low-affinity binding sites for Ru. When ascites hepatoma cells were incubated with Ru after incubation with a low concentration of papain, most of the Ru was not bound to the cells but was found in the medium containing solubilized glycoproteins. However Ru bound mainly to washed cells after the incubation with papain. About 65 % of the Ru bound to ascites hepatoma cells was liberated by the papain treatment, and about 45 % of the liberated Ru was precipitated by cetyltrimethylammonium bromide, indicating that Ru bound tightly to glycopeptides. These results suggest that the tumor affinity of (103)Ru is related to specific binding to glycopeptides on the tumor cell surface.