-13- SUMMARY Age related and/or radiation-induced age effects on immunological | competence were tested in a Marshallese population of about 150 people, 50 of whom had been exposed to fallout radiation in 1954, In the unexposed group decreasing immunological competence with aging was indicated by : (1) decreasing percentage transformation of peripheral blood lymphocytes . by phytohemagglutinin stimulation indicating decreasing immunological competence of lymphocytes; (2) significant increase in gamma globulin reflected also in increases in IgG, IgA immunoglobulins and also increase of K light chains. These latter findings are believed to be associated with age-accumulated effects of repeated exposures to infectious agents and perhaps to increased autoimmune reactions. In the irradiated population there were certain significant, differences in the findings compared with the unexposed population which may indicate radiation effects. A relative depression of platelet and neutrophil levels of the peripheral blood was noted. Some slight degree of depression of peripheral blood elements has, however, been noted since exposure. Also noted was reduced gamma globulin levels (also IgG, IgA and K light chains on immunodiffusion analysis), more pronounced in older age groups which may indicate a radiation-induced aging effect, Evidence, however, for relative loss of immunological capacity in the exposed population has not been evident ; ’, based on incidence or severity of diseases compared with the unexposed population