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Immunologic parameters were studied among survivors of the 1945 atomic bombs who now reside in the United States. Of all known survivors living in the U.S., about 40% (n = 189) participated in this study. Of those survivors on whom radiation exposure information was available (n = 168), 96% were exposed to less than 50 rad at the time of the bomb (ATB). Survivors were divided into two groups; those exposed to varying low doses of radiation (S+ group, exposed at less than or equal to 2500 m from the hypocenter) were compared with those exposed to "O rad" (S0 group, exposed at greater than 2500 m from the hypocenter). Of the former group, 92% were exposed to less than 100 rad and 89% to less than 50 rad ATB. Cellular immune responses, including natural cell-mediated cytotoxicity (NCMC), interferon production, and the mitogenic response to PHA, tended to be higher among S+ individuals, although only the difference for NCMC was statistically significant. This was suggestive of a trend which was consistent with the higher serum interferon levels and lower frequencies of detectable immune complexes and antimitochondrial antibodies among the S+ group, although these differences were not statistically significant. Other immunologic parameters which showed no trend included frequency of antinuclear antibodies, rheumatoid factor, levels of serum immunoglobulins, levels of isoantibodies and heteroantibodies, and the magnitude of the mixed lymphocyte reaction.