Clinical tumor targeting studies with monoclonal antibody (mAb) G250 showed excellent targeting of primary renal cell carcinomas (RCC). However, tumor uptake decreased at higher mAb G250 doses, suggesting saturation of the G250 antigenic determinants. In this immunohistochemical study we investigated the saturability of G250 antigen sites after i.v. administration of mAb G250 at various protein doses in nude mice with RCC xenografts. Five groups of mice received five different protein doses (1, 3, 10, 30 or 100 micrograms) of murine mAb G250. Three days post injection mice were killed and the tumors were removed. Free G250 antigen sites, i.e., not targeted by the i.v. injected murine mAb G250 were determined by immunohistochemical staining with chimeric mAb G250. Distinct staining of the G250 antigen was observed only at the 1 and 3 micrograms dose whereas G250 antigen staining at higher doses was virtually negative. The results of this study indicate that saturation of antigen occurs at relatively low doses of i.v. administered mAb G250. Apparently, all antigenic determinants present on the RCC tumor cells were targeted, while previous preclinical studies suggested that i.v. administration of mAb G250 only saturated the accessible antigen sites.