As a model system to study protein traffic in polarized epithelial cells, we have used the polymeric immunoglobulin receptor. This receptor travels first to the basolateral surface, where it can bind polymeric IgA or IgM. The receptor is then endocytosed and delivered to endosomes. The receptor is sorted into transcytotic vesicles, which are exocytosed at the apical surface. The 103-amino acid cytoplasmic domain of the receptor contains several sorting signals. The 17 residues closest to the membrane are an autonomous signal that is necessary and sufficient for basolateral sorting. For rapid endocytosis there are two independent signals, both of which contain critical tyrosine residues. Finally, transcytosis is signaled by phosphorylation of a particular serine.