The 30-day median lethal dose of H32O for CF1 female mice is about 1 mc/g. body wt. when given in a single injn. The estimated radiation dosage is about 800 rep in the first 3 days and about 1000 rep in the first 12 days after injn. It is concluded that no unexpected factor of biological effectiveness exists for tritium beta rays in relation to X or gamma rays.