The toxicity of the alpha particle emitting halogen astatine-211 was examined in male and female mice. Pathological changes were seen in mice killed at 14 days and/or at 56 days following a single injection of 61 kBq211 At per g body weight. The tissues affected, in order of severity were: spleen, lymph nodes, bone marrow, gonads, thyroid, salivary glands and stomach.