Abstract: The present study was carried out to investigate the possibility of Mentha crispa (Mint; M. crispa) extract being capable to alter the labeling of blood elements with Technetium-99m (99mTc). Blood was incubated with M. crispa extract. Stannous chloride solution and 99mTc, as sodium pertechnetate, were added. Blood was centrifuged and plasma (P) and blood cells (BC) were isolated. Samples of P and BC were also precipitated, centrifuged and insoluble (IF) and soluble (SF) separated. The percentage of radioactivity (%ATI) in BC, IF-P and IF-BC was calculated. Histological evaluations were performed and the morphology of the red blood cells was observed under optical microscopy showing important morphological alterations on the shape of the RBC treated with M. crispa extract. The %ATI decreased: on BC from 97.3 ± 1.92 to 60.0 ± 2.44; on IF-P from 74.8 ± 3.78 to 9.99 ± 3.61; on IF-BC from 88.6 ± 5.41 to 58.4 ± 11.55. The substances of the M. crispa extract could increase the valence of the tin ion from stannous (+2) to stannic (+4). This fact would decrease the %ATI on blood elements and would indicate the presence of oxidant agents in the M. crispa extract with possible generation of free radicals.