The transcytotic pathway allows for the bidirectional transport of endocytosed solutes, lipids, and proteins between the two membrane domains of polarized epithelial cells while maintaining the functional integrity of the epithelial tissue. A method is described to measure basolateral-to-apical transcytosis of immunoglobulin A (IgA) in polarized Madin–Darby canine kidney (MDCK) cells expressing the polymeric immunoglobulin receptor (pIgR). The cells are grown on porous Transwell filter supports, and radiolabeled ¹²⁵I-immunoglobulin A (IgA) is internalized from the basolateral pole of MDCK cells. During a subsequent 2-h chase, the amount of ¹²⁵I-IgA that is recycled, degraded, or transcytosed is quantified. This assay can be adapted to follow the postendocytic fate of other ¹²⁵I-labeled ligands and proteins.