Transfer of cyclosporine A (CsA) in the human placenta has been studied in vitro using a dual perfusion technique of isolated placental lobules. Antipyrine was used as a reference marker for comparison. Our studies showed a negligible transfer of CsA from the maternal to the fetal circulation, representing less than 5% of the maternal drug load. These results are in agreement with that reported earlier in an in vivo study in a pregnant woman. We conclude that the low placental transfer of CsA is attributable mainly to the drug’s relatively high molecular weight.