The concentrations of three immunoglobulins (IgG, IgA, and IgM) were studied in sera of 307 healthy children from the age of one month to 16 years. The method used was a single radial immunodiffusion technique, with the use of commercially available antibody agar plates. The results obtained were compared with those found in umbilical cord sera and with the average levels in healthy adults. The average concentration of IgG was lowest at the age of 3 months, i.e. about 40 percent of adult level. At the age of one year the average level of IgG was 50 percent of adult level and approached the adult level at the age of 6~8 years (Fig. 1,4 and 5 and Table 1). In cord sera IgA could only be demonstrated in three neonates out of 20. At the age of one year an average IgA level of about 25 percent of adult level was found and the adult level was reached at age of 12 to 16 years(Fig. 2, 4 and 5, and Table 1). IgM was detected in all cord sera with an average concentration of 15 percent of adult level. At the age of 3 months the IgM values were about 50 percent of adult level and at the age of one year approximately 70 percent(Fig. 3,4 and 5, Table 1).