Epstein-Barr virus (EBV) is distributed widely throughout the world. Apart from a association with two geographically-restricted malignancies (Burkitt's lymphoma and nasopharyngeal carcinoma), EBV is thought to be implicated in the etiology of B-cell lymphoma in immunocompromised individuals. In these patients, monitoring the viral load in serum can provide useful information on the timing of the instigation of antiviral therapy, i.e. as soon as a rise is detected. PCR technology, owing to its high sensitivity, is used frequently in such situations. In order to gain further insight into the nature of the peripheral blood cells carrying the viral genome on a cell-by-cell basis, an in situ amplification technique was developed as a model using two cell lines growing in suspension, with the aim of distinguishing between EBV-positive and EBV-negative cells. Preliminary experiments were undertaken subsequently on clinical samples from patients with infectious mononucleosis and patients with lymphoma indicating that this technique might be useful clinically.