A porcine respiratory coronavirus ( prcv) was inoculated by aerosol into nine hysterectomy-derived and colostrum-deprived pigs at the age of one week. They were killed at different times after inoculation and tissues were sampled for virus isolation and immunofluorescence. Results indicate that virus replicated to high titres in the respiratory tract. Replication mainly occurred in alveolar cells but also in epithelial cells of nasal mucosa, trachea, bronchi, bronchioli, in alveolar macrophages and in tonsils. After primary replication in the respiratory tract, viraemia occurred. Virus also reached the gastrointestinal tract after swallowing. Subsequently, PRCV was observed to replicate in the ileum. The infection spread within a few days from the ileum to the duodenum. Replication in the small intestine remained limited to a few cells located in or underneath the epithelial layer of villi and, or, crypts. The cell type could not be identified. Virus was isolated from mesenteric lymph nodes in all pigs, but immunofluorescence was not observed. Results show that small changes in molecular structure between transmissible gastroenteritis virus and prcv resulted in important changes in host cell tropism.