The polymerase chain reaction (PCR) was used to study DNA extracted from the blood of 25 transplant patients, 5 patients with infectious mononucleosis, and 13 healthy subjects and autopsy or biopsy tissue from 29 patients with lymphoproliferative disorders. Primers were directed to conserved regions of the Epstein-Barr virus (EBV) genome encoding capsid protein gp220 and Epstein-Barr nuclear antigen 1. Specific EBV amplification was found in the blood of 11 of 25 transplant patients and all patients with infectious mononucleosis. All patients with lymphoproliferative disorders occurring in the presence of immunosuppression (eight organ transplant patients and two patients with acquired immunodeficiency syndrome) had biopsies positive for EBV by PCR. Only 1 of 19 samples from lymphomas or leukemias unrelated to immunosuppression contained EBV. PCR confirmed the very close association of EBV and lymphoproliferative disorders occurring in the presence of immunosuppression. The significance of detecting EBV sequences in the blood of transplant patients, particularly in relationship to lymphoproliferation, requires further study.