The serum levels of immunoglobulins at various times throughout pregnancy were measured in 11 healthy women. The concentrations of IgG, IgA, and IgM decreased significantly in the second and third trimesters, the mean decreases at the second trimester being 18, 13, and 9%, respectively. When the decreases were expressed on the basis of scrum total protein, the decreases in IgG and IgA were significant but the decrease in IgM was not. The level of IgE either decreased or increased during pregnancy. Maternal age, emesis, ABO-incompatibility, and the sex and weight of the baby at birth were not related to the initial concentration or to the extent of decrease of immunoglobulins during pregnancy. In a case of Rh incompatibility, increase of immunoglobulins was observed concomitantly with the transient appearance of anti-Rh(D) antibody. Immunoglobulin depletion in pregnancy seems to result from both immune suppression and hemodilution.