used for quantifying the serum immunoglobulins in antibody-agar plates has been previously described (Fahey & McKelvey, 1965). Peripheral blood elements. - The enumeration of peripheral blood elements was part of the routine medical examinations of the Marshallese.. (Richar & Breakell, Leukocyte counts 1959) were carried out electronically (Coulter A counter). Platelet counts (Bull, Schneiderman & Brecher, 1965) were done by phase microscopy. Differential counts of leukocytes (200 cells) were performed on Wright stained smears. Hematocrits were determined by the micro-capillary method (Strumia, Sample & Hart, 1954), and sedimentation rates by the method of Wintrobe (1: Statistical analysis of data. - An analysis of variance was used to determine ‘differences among groups for age, sex and radiation exposure. programmed and analyzed on a high speed digital computor. These data were Since sex differences were not apparent the sexes were combined and each criterion analyzed for age correlation (r value). The level of significane (p) of differences in the group exposed to fallout. compared with the unexposed. group (radiation effects) was expressed. | RESULTS The results are summarized in Table 2, and the values of the various criteria are plotted as a function of age in figures 1 through 4, -Most of the changes noted generally reached a maximum effect in the 40-50 year age group with little further change in the older groups. Therefore ‘ most of the age-dependent correlation was due to differences between the younger (15-50 years) and older age groups (beyond 50 years of age). Response oflymphocytes to phytohemagglutinin stimulation, The transformation of lymphocytes into blast forms as a result of PHA stimulation in peripheral blood cultures showed a decreasing response with increasing age (Fig. 1) which