Aminopeptidase N is a species-specific receptor for transmissible gastroenteritis virus (TGEV), which infects piglets, and for the 229E virus, which infects humans. It is not known whether these coronaviruses are endocytosed before fusion with a membrane of the target cell, causing a productive infection, or whether they fuse directly with the plasma membrane. We have studied the interaction between TGEV and a cell line (MDCK) stably expressing recombinant pig aminopeptidase N (pAPN). By electron microscopy and flow cytometry, TGEV was found to be associated with the plasma membrane after adsorption to the pAPN-MDCK cells. TGEV was also observed in endocytic pits and apical vesicles after 3 to 10 min of incubation at 38 degrees C. The number of pits and apical vesicles was increased by the TGEV incubation, indicating an increase in endocytosis. After 10 min of incubation, a distinct TGEV-pAPN-containing population of large intracellular vesicles, morphologically compatible with endosomes, was found. A higher density of pAPN receptors was observed in the pits beneath the virus particles than in the surrounding plasma membrane, indicating that TGEV recruits pAPN receptors before endocytosis. Ammonium chloride and bafilomycin A1 markedly inhibited the TGEV infection as judged from virus production and protein biosynthesis analyses but did so only when added early in the course of the infection, i.e., about 1 h after the start of endocytosis. Together our results point to an acid intracellular compartment as the site of fusion for TGEV.