AbstractThe fate of native alpha 2‐macroglobulin (α2M) or its trypsin complex (α2M‐T) was studied in the isolated dually‐perfused lobule of term human placenta. [125I]‐α2M added to the maternal circuit was unchanged during the course of the perfusion with minimal activity becoming associated with the placental tissue. Transfer of radioactivity into the fetal circulation accounted for only 0·07 per cent of the initial dose after 2 h. In contrast, [125I]‐α2M‐T was rapidly taken up into the placental tissue (nearly 28 per cent of the initial dose during the 2‐h perfusion) and breakdown products were released into both maternal and fetal circulations. At the end of 2 h, radioactivity levels on the fetal side were 13 times higher than those found with the native protein. These indications of a classical receptor‐mediated uptake and breakdown pathway were confirmed in experiments in which the acidotrophic agent chloroquine was added to the maternal circuit prior to the α2M‐T. In the presence of chloroquine, tissue uptake was inhibited and the subsequent release of radioactive degradation products into the fetal circuit was similar to the levels seen with α2M. Incubation of term trophoblast cells at 37°C with [125I]‐α2M‐T revealed over three‐fold greater cell‐associated activity than was found with the native protein. In another series of experiments, a purified microvillous membrane fraction was prepared from term placentae using buffers containing 1 mMiodoacetate. In the presence of this proteolytic enzyme inhibitor, binding studies showed a single class of low affinity receptors for the α2M‐T complex capable of binding 4·8 ± 1·3 (SEM) μg of complex per mg of membrane protein. T