Using isolated rat seminiferous tubules as an in vitro model, we have found that 2 3 8 Pu can cross the blood-tubule barrier and accumulate within tubules in a time dependent manner. Furthermore, similar to 5 9 Fe, tubule 2 3 8 Pu uptake was inhibited by the addition of excess transferrin, suggesting that plutonium may utilize the physiological iron-transferrin pathway to cross the blood-tubule barrier. However unlike 5 9 Fe, 2 3 8 Pu was only transiently associated with the tubules, suggesting differences in the intracellular processing of these radionuclides. The assumptions made in the estimation of doses to the human testis from incorporated plutonium are considered.