homozygous for the alternate allele (genotype Au/Au) or heteronygotes, (Aut /Au) do not have detectable Australia antigen(phenotype Au dy (4) Although Au (1) has not been found in normal United States populations, it is relatively common in some forms of leukemia, i.e., 18% of acute myelo- genous leukemia, 17% of chronic lymphocytic leukemia. It has not as yet been found in chronic myelogenous leukemia. A consideration of the United States data alone, therefore, would suggest that Australia antigen is a leukemiaspecific antigen and may be associated with an increased risk of developing leukemia. It has also been found in high frequency (approximately 30%) in patients with Mongolism (Down's syndrome), a disease characterized by a high risk of developing leukemia. (5) During our studies in Cebu, we found that Australia antigen is significantly more common in patients with lepromatous leprosy than those with tuberculoid leprosy or non-leprosy individuals from the same area. Consideration of all the patient data suggests that a common factor in the various disedses in which the trait is found is a deficiency of the immunological system associated in some as yet unexplained manner with the presence of Australia antigen. Since the Rongelap people will be medically examined for many years, it will be of considerable interest to see if there is a differential susceptibility to disease, particularly leukemia, in respect to Australia antigen. In several populations, including Marshall Islanders, a significant decrease in Australia antigen frequency with age has been detected. This may be due to selection against individuals with this trait, presumably compensated for by some, at present unknown, selective advantage in respect to one of the genotypes. References L. Blumberg, B.S., Alter, J.J., and Visnich, S., A "new" antigen in leukemia sera. J. Am. Med. Assoc., 191: 541-546, February, 1965. 2. Alter, H.J., and Blumberg, B.S., Studies on a "new" human isoprecipitin system (Australia antigen). Blood, 27(3): 297-309, 1966. 3. Melartin, L. and Blumberg, B.S., Production of antibody against "Australia antigen" in rabbits. Nature, 210: 1340, 1966. 4. Blumberg, B.S., Melartin, L., Guinto, R.A., and Werner, B., Family studies of human serum isoantigen system (Australia antigen). Am. J. Human Genetics,in press, 1966. 5. Blumberg, B.S., An inherited serum isoantigen in leukemia and Down's syndrome. J. Clin. Invest., 45: 988, 1966. 6. Blumberg, B.S. and Melartin, L., Conjectures on inherited susceptibility 900843 0 to lepromatous leprosy. Intern. J. Leprosy, 34(1): 60-64, 1966.