Abstract: We have evaluated the influence of the Ginkgo biloba extract (infusion and crude extract) on the labeling of red blood cells (RBC) and plasma proteins with technetium-99m (Tc-99m). We also studied the morphometry of the RBC treated with Ginkgo biloba extract (EGb). Blood was withdrawn and incubated with EGb. Stannous chloride was added and, then, Tc-99m was added. Plasma (P) and RBC were isolated, also precipitated with trichloroacetic acid and soluble (SF) and insoluble fractions (IF) separated. The morphology of the RBC also was evaluated under optical microscope and morphometry. The analysis of the results shows that there is a decrease in the radioactivity on RBC and on IF of P and of RBC with the EGb. The study of the morphology of RBC showed important morphological alterations due to treatment with EGb. These observations were confirmed by morphometry. We suggest that the chemical agents presents in the Ginkgo biloba extract or its active metabolites could act, with: (i) a chelating action of the ions stannous/pertechnetate or (ii) by damages induced in plasma membrane; (iii) or by competition of the cited ions to the same bindings sites; or (iv) with possible generation of reactive oxygen species that could oxidize the stannous ion.