This study demonstrates that IgG transfer in vitro across the isolated perfused human placental lobule can be successfully studied by using natural forms of IgG. The transfer of anti-RhD IgG (anti-D) was measured in the presence and absence of intravenous immunoglobulin (IVIgG). When anti-D and IVIgG were present alone each crossed the placenta at about the same rate, but when both forms were present at the same time the movement of one interfered with the movement of the other. This pattern of transfer is consistent with receptor-mediated transcytosis. The interactions of IgG with trophoblastic transporters may therefore be studied without the complications that might arise from the use of conventionally labelled molecules.