Thus several indications have been seen of impairment or borderline deficiency of the immunological status in the exposed Rongelap people in earlier years, but no evidence that such deficiency was related to disease incidence, with the possible exception that the increased development of thyroid malignancy in the exposed Rongelap people may be an indication of reduced immuno- logical surveillance. Recent rises of leukocyte and gamma globulin levels to control values indicate some degree of recovery, but better tests for immunological status are needed. C. Chromosome and Genetic Studies 1. Chromosome Studies In 1964 chromosome preparations were obtained from lymphocytes cultured from the peripheral blood of 43 exposed (21 age <20; 22, age >20) and 8 unexposed Rongelap people (65). Chromosome aberrations were noted in 23 of the exposed and in 5 of the unexposed Marshallese, but the exposed group had a num- ber of two-break aberrations (represented by dicentric chromosomes, translocations, and a ring form) that are thought to be associated with radiation exposure. No two-hit aberrations were found in the unexposed group, but both groups had an unusual number of acentric fragments, the cause of which is not known. Paradoxically, the lower-exposure Ailingnae group had more aberrations than the Rongelap group who had a higher exposure. These studies indicate that a small but significant number of chromosome aberrations persisted in blood lymphocytes in some Marshallese as late as 10 years after exposure. The results are consistent with those of similar studies on the exposed Japanese fishermen (66), on victims of other radiation accidents (67), and on Japanese bomb survivors (68). 2. Isoleucine Misincorporation in Hemoglobin A of Marshallese* Adult human hemoglobin A has no coded isoleucine; thus, the presence of isoleucine in hemoglobin A must result from errors in transcription or translation or from somatic mutations (1,35,37). Between 1974 and 1978, analyses were made of the isoleucine content (35) in hemoglobin A of 52 exposed Marshallese, 4 of whom were exposed in utero; 25 sex- and age-matched con- trols; and 5 children born after 1954, one or both of whose parents had been exposed. The frequency of isoleucine substitution for other amino acids in hemoglobin A was calculated by dividing the nanomoles of isoleucine by the total nanomoles of all other amino acids in each sample. The average frequen- cies are shown in Table 1. Values >5x107? are considered to be above the normal range. Among 25 controls, 4 showed values >5x1079, but repeat analyses on two of these, the following year, gave values <5x107>. Among 48 persons ex- posed when 1 to 30 years of age, 13 showed isoleucine substitution frequencies >5x1079. Values for 11 of them were between 5.85 and 19.79xl072. As a group, only the persons exposed to 175 R had values significantly different from *Drs. R.A. Popp, E.G. Bailiff, and C.P. Hirsch (Biology Division, Oak Ridge National Laboratory, Oak Ridge, TN). aA - 33 -