We report here a 33-year-old patient found at 20 weeks' gestation to be anemic and to have both a serum M-protein (immunoglobulin [Ig]G kappa) and a kappa-type urinary Bence Jones protein. The concentration of these proteins, measured during pregnancy and in the postpartum period, demonstrated the presence of the maternal serum IgG kappa component in the amniotic fluid and its transmission to, and eventual disappearance from, the newborn.