Abstract Immunoglobulin levels in 8 hydropic fetuses were studied. Six examples were paired with sera from the mother. The remaining 2 were from fetuses only. Specimens from 37 normal mother-neonate pairs were analyzed as a control group. The maternal immunoglobulin G (IgG) levels of hydropic fetuses were within the normal range. While the levels of IgG of all cord blood were very low, normally these should be at the level equal to those of maternal sera in a normal condition. These remarkably low levels of IgG of cord sera suggest a reduced ability of placental transfer of globulin in this particular pathologic condition. Two hydropic fetuses from syphilitic mothers showed detectable levels of immunoglobin M (IgM) in cord blood. Besides these 2 fetuses from syphilitic mothers, the IgM level of cord sera in all 6 of the other fetuses was below the measurable level, suggesting an etiology of a noninfectious nature, but failure of the immune response of the hydropic fetus to certain types of infection could not be ruled out entirely.