Quantitative determination of serum immunoglobulins was carried out by radial diffusion technique in 17 mothers and 43 cord blood samples. Mean maternal serum IgG, IgM and IgA were 1471 +/- 349 mg%, 109 +/- 21 mg% and 240 +/- 62 mg% respectively, and these were equivalent to maternal levels in Thai adults. Mean umbilical cord IgG was 1571 +/- 298 mg% and IgM was 10 +/- 5 mg%. IgA was not detected in the cord blood. There were no differences in the immunoglobulin levels between male and female infants. The IgG levels in the maternal sera and those in the cord sera of the corresponding offsprings were almost identical, but those of IgM were markedly different, suggesting a placenta barrier of this latter immunoglobulin. Thus, IgM level above the normal cord blood suggest intrauterine infection.