Study of the labeling of Schistosoma mansoni cercaria with technetium-99m (99mTc) at room temperature (25 degrees C) and 37 degrees C shows that the incorporation of radioactivity in this cercaria increases with the increase in stannous chloride concentration, reaching a constant value threshold at 130.00 microM. Strong binding of the 99mTc was obtained since the radioactivity was not washed out. The characteristic motion of the cercaria, labeled at 25 degrees C and 37 degrees C, was only modified at the concentration of 1300 microM for both temperatures, showing that the methodology here described can be applied to living structures. With this technique it is possible to obtain a cercariae suspension labeled with a radionuclide that is very inexpensive and readily available. Furthermore, it is a gamma emitter with a photon energy of 140 keV that would permit one to make scannings of the infected animals and causes less of an environmental impact and present fewer radioactive disposal problems than the long lived radionuclides.