Blood constituents labeled with technetium-99m (99mTc) have been used in nuclear medicine procedures and drugs are capable to interfere on this labeling. Lantana camara (lantana) has medicinal properties and it has been used in folk medicine. The aim is to verify the effect of a lantana extract on the labeling of blood constituents with 99mTc. Blood of rats was incubated with extract, stannous chloride and 99mTc, as sodium pertechnetate. Plasma (P) and blood cells (BC) were isolated, also precipitated with trichloroacetic acid and soluble (SF) and insoluble fractions (IF) were separated. The % of radioactivity (%ATI) in these samples was calculated. Samples of labeled BC were washed and the %ATI maintained (%ATI-M) in the BC was determined. The results showed that lantana extract decreased significantly (p < 0.05) in the IF-P from 70.24 +/- 2.59 to 11.95 +/- 3.07. This effect was not observed in the BC and IF-BC. The BC-%ATI-M was significantly (p < 0.05) decreased in all concentrations tested when the BC was washed. This fact was not observed in the control. Substances present on the extract should have redoxi action decreasing the concentration of the stannous ion and this condition could justify the effect on the IF-P. The results about the BC-%ATI-M should indicate a possible effect on the transport of ions through the erythrocyte membrane.