We used slot blot hybridization, Southern blot hybridization, and in situ hybridization to investigate the presence of Epstein-Barr virus (EBV) genomes in biopsy tissues from patients with Hodgkin's disease. Slot blot hybridization performed on DNA of tissue specimens from 16 patients revealed that biopsy tissue from 3 (19 percent) contained EBV DNA. Southern blot hybridization with a DNA probe containing the 500-base-pair tandem repeated sequences located at the termini of the EBV genome confirmed the findings of the slot blot hybridization in the three positive tissue specimens and indicated the monoclonality of the EBV-infected cells in such tissues. In situ hybridization performed on the three positive specimens and on two from a previous study localized EBV nucleic acid to the Reed-Sternberg cells and variants in all specimens, with intense hybridization to Reed-Sternberg cells in two, less intense but consistent hybridization to Reed-Sternberg cells in two, and focal hybridization to Reed-Sternberg cells in one. We conclude that EBV genomes are present within Reed-Sternberg cells and variants in some patients with Hodgkin's disease and that the infected cells are monoclonal.