WE have determined the IgG, IgA, and IgM levels of 315 normal adults and 503 children of various ages¹. To find out whether differences exist between males and females, we compared the levels for the sexes within various age groups, breaking these down into “fine” and “coarse” subgroups. There were fourteen fine groups of about thirty children each and five coarse groups of about 100 children each. The subjects were laboratory personnel, blood donors, college students, school-children, babies, and inmates of a home for recently abandoned children, all apparently healthy. Immunoglobulins were determined by a single radial immunodiffusion method² using commercial antibody-agar plates standardized against purified immunoglobulins. The standardization curves used were the least-squares linear regressions of log mg/100 c.c. versus the log ring diameter. This relationship fitted the results better than the semilogarithmic one used by others^2–4. The males and females in each group were compared by Student's t test. We used log mg/100 c.c. for the t tests and to calculate the means, because the distributions of the three major immunoglobulins among adults are approximately log normal.