The basic objective of this work is to correlate placental transport phenomena with the cytological and cytochemical organization of the placenta. The specific objectives are: (1) to study by electron microscope cytochemistry the absorption and transport of tracer proteins and antibodies by the guinea pig and human placentas using enzyme-labeled antibodies; (2) to study the energy requirements underlying placental absorption by means of metabolic inhibitors; (3) to study enzyme localization in the placenta using electron microscopy and cytochemistry; (4) to investigate the comparative fine structure of the mammalian placenta.