Pregnant patients receive immunoglobulin preparations for the prevention of connatal viral infections of the newborn in the case of impending or manifest maternal infections. In the past, class G immunoglobulin preparations could only be given intramuscularly because of possible antigen independent complement activation by the Fc-part of the IgG molecule when administered intravenously. The I. M. application results in a delay in reaching maximum serum levels, and the levels are much lower compared to I. V. application. Today, I. V. tolerable IgG preparations exist because methods have been developed either to remove or to modify the IgG Fc-part. As the Fc-part is necessary for the placental transport of antibodies, the above mentioned manipulation can drastically impair the transport. In this study, an anti-HBs seronegative pregnant patient received an anti-HBs IgG preparation (greater than or equal to 500 IU anti-HBs) intravenously. These IgG molecules are rendered I. V. tolerable by modification of the Fc-part with beta-propiolactone (beta-PL). At delivery, the newborn's serum anti-HBs concentration was 13 mIU per ml, the mother's 35 mIU per ml. This result clearly demonstrates that IgG molecules which have been modified by treatment with beta-PL are transported from the circulation of the mother across the placenta to the fetus. The role of intravenously tolerable hyperimmunoglobulins with special respect to beta-PL treated preparations for the prevention of connatal infections is discussed.