Abstract: The biological toxicity of uranium (in the form of uranyl nitrate)and tech-netium (in the form of ammonium pertechnetate)are studied, using an unicellular organism tetrahymena pyriformis S1 as a biological model which has been widely used in toxicological testing.The proliferation of T. pyriformis in a culture medium of 0.5% d-glucose and 1-2% polypeptone containing small amount of uranium or technetium is observed.lt is concluded that when the concentration range of uranium in the medium between 0.02-20 ppm and 0.1-100 ppm for Tc, no obvious influences are revealed.When U>100 ppm and Tc>500 ppm, the inhibitions of growth of T. pyriformis induced by U and Tc slightly appear. But even their concentrations increase up to 1200 ppm, the cells can still survive.Therefore, it seems reasonable that both uranium and technetium are considered as low toxic elements.These findings would be meaningful in respect to the applications of uranium and technetium in various fields.