o,) homozygous for the alternate allele (genotype Au/Au) or heterozygotes )Au ype (phenot (Aul/Au) do not have detectable Australia antigen ions, Although Au (1) has not been found in normal United States populat myeloacute of 18% it is relatively common in some forms of leukemia, i.e., yet been genous leukemia, 17% of chronic lymphocytic leukemia. It has not as States United the of found in chronic myelogenous leukemia. A consideration aleukemi a is antigen data alone, therefore, would suggest that Australia ing develop of risk ed specific antigen and may be associated with an increas in 30%) imately (approx cy frequen leukemia. It has also been found in high high a by erized charact disease a e), patients with Mongolism (Down's syndrom 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 diseases in which the trait is found is a deficiency of the immunological system associated in some as yet unexplained manner with the Since the Rongelap people will be medically presence of Australia antigen. 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 1. 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 4. Blumberg, B.S., Melartin, L., Guinto, R.A., and Werner, B., Family studies antigen" in rabbits. Nature, 210: 1340, 1966. of human serum isoantigen system (Australia antigen). Genetics,in press, 1966. Am. J. Human 5. Blumberg, B.S., An inherited serum isoantigen in leukemia and Down's 6. Blumberg, B.S. and Melartin, L., Conjectures on inherited susceptibility to lepromatous leprosy. Intern. J. Leprosy, 34(1): 60-64, 1966. syndrome. J. Clin. Invest., 45: 988, 1966.