The polymeric immunoglobulin receptor (pIgR) has been proposed as a therapeutic target, but its potential depends on the efficiency of uptake and trafficking of the receptor ligand. Mouse monoclonal antibodies (Mabs) directed against pIgR, selected for strong binding to secretory component (SC) and secretory IgA (sIgA) were tested in a transcytosis assay in 16HBEo- cells (human bronchial epithelial cell line) transfected with human pIgR. Intracellular trafficking was followed by confocal microscopy. Mabs fell into two classes. For two Mabs, transcytosis from basolateral to apical surface is rapid, unidirectional, and little Mab is retained in the cell. For three Mabs, basolateral to apical transcytosis occurs to a significantly lesser extent, reverse transcytosis is permitted, and some of the Mab is retained in the perinuclear region even after 24 hr. When tested for their ability to recognize and immunoprecipitate pIgR with systematic truncations and deletions of the 5 immunoglobulin (Ig) like domains, all Mabs bound to the 5^th Ig like domain, but three of them also bound to the C terminal region of pIgR near the plasma membrane. Different binding sites probably account for the different trafficking of these Mabs and may predict differential therapeutic utility.