Many hormones, growth factors, and other large molecules bind to speci- Jic receptors on the surface of eukaryotic cells and are rapidly taken into these cells. Current techniques of protein purification have made available sufficient amounts of these molecules so that detailed studies of their interaction with cells could be carried out. These studies have been performed on just a few types of cells, but it is clear that all types of cells carry out a_ similar internalization process. The realization that cells rapidly internalize hormones, growth factors, transport proteins, toxins, and...