An in situ polymerase chain reaction (IS-PCR) technique was used to detect and differentiate strains of episomal Epstein-Barr virus (EBV) in infected cells. IS-PCR was performed on cell monolayers in eight-chamber glass slides using EBV type-specific primer pairs conserved within the EBV-encoded nuclear antigen (EBNA) 3C region. The amplicons in the cells were detected by in situ hybridization using EBV type-1 and type-2 specific 5'-biotinylated oligonucleotide probes and avidin-conjugated alkaline phosphatase as secondary reagent. This method was successfully used to identify EBV strains not only in Burkitt's lymphoma cell lines but also in B cells obtained from a patient with infectious mononucleosis. The technique described on this report is a reliable method to detect latently infected EBV-positive cells and can potentially be used to identify and type EBV strains present in clinical specimens.