While considering the geological disposal of radioactive wastes, the behaviour of the radionuclide Np and its daughter element Pa was investigated in the presence of a mixture of anaerobic bacteria (MAB). Originally, MAB were used for the treatment of pulp and paper wastewater. The interaction between radionuclides and bacteria was evaluated by determining distribution coefficients (Kd) over 10 days and at 5 degrees C and 35 degrees C. Kd for Np at 35 degrees C after 5 days had a low value around 10(-2) After 10 days, however, Kd was > 100-fold higher. On the other hand, Kd at 5 degrees C was low (10(-2)) throughout, without any significant increase over time. The interaction between Pa and MAB was found to be stronger than that for Np, with Kd for Pa about 100 times higher. The Kd was controlled by some basic factors, the activity of MAB, the complexing capacity of MAB, and the chemical conditions in the solution such as pH and Eh.