Replication of porcine reproductive and respiratory syndrome virus (PRRSV) was studied in formalin-fixed paraffin wax-embedded lung tissues from seven naturally infected piglets by in-situ hybridization with a non-radioactive digoxigenin-labelled probe. A 433 base pair cDNA probe for the viral RNA encoding the nucleocapsid proteins of a Korean PRRSV isolate was generated by the polymerase chain reaction. All seven piglets infected with PRRSV showed a distinct, positive signal, scattered throughout the alveolar septa and spaces. Positive cells typically exhibited dark brown staining deposits in the cytoplasm without background staining. In-situ hybridization demonstrated that PRRSV replicated primarily in interstitial and alveolar macrophages, and occasionally in type 2 pneumocytes. The bronchial or bronchiolar epithelium did not exhibit a hybridization signal for PRRSV nucleic acids. The anterior and middle lobes of the lung were more reliable than the caudal or accessory lobes for the detection of PRRSV nucleic acids. The in-situ hybridization technique used was rapid, specific and sensitive, and may prove useful for the diagnosis of PRRSV infection in routinely fixed and processed tissues.