Immunoglobulin values were determined in carefully selected healthy, normal children of different gestational and chronological ages. Criteria for "normality" were eutrophy (length, weight, head circumference) and absence of obvious malformation, infection or other disease. Immunoglobulin concentrations were measured in radial immunodiffusion, calibrated by a standard serum according to WHO recommendations. Since the distribution patterm of the individual values was found to be asymmetric in certain age groups, percentiles were calculated. They represent a physiologically more meaningful pattern than the arithmetic mean with standard deviation. Especially at the critical chronological age around 3 months the low values are closer together and the high values wider spaced than is the case with the standard deviations. In premature infants the transplacentally acquired IgG supplement is reduced in inverse relation to the duration of gestation. However, the IgG levels of all prematures equal those of term babies at the chronological age of 14 weeks. IgM can, in contrast, be adequately synthesized by all age groups within the first week of life.