The concentrations in cord blood of total immunoglobulin G (IgG) and the four subclasses of IgG were measured in 34 fetuses at a mean gestational age of 25 weeks (range, 18 to 35 weeks). The blood samples were obtained by percutaneous umbilical blood sampling, and results were compared with the respective IgG subclass concentrations of the mothers. The efficiency of transplacental transfer of the different IgG subclasses was determined. Transfer of IgG1 and IgG4 was found to be significantly more efficient than that of IgG3 and IgG2. IgG2 was the subclass least efficiently transferred from mother to fetus. These differences may partly explain the susceptibility of newborns to various pathogens, such as streptococcus group B.