The Epstein-Barr virus (EBV) is closely related to Hodgkin's disease (HD), while the BCRF-I (viral [v] IL-10) gene of the EBV is highly homologous to the human interleukin-I0 (h IL-10) gene. To investigate the relationship of IL-I0 and HD, we performed both immunostaining and in situ hybridization (ISH) in 30 cases of HD. The presence of EBV in Hodgkin (H) and Reed-Sternberg (RS) cells was seen in 16 of the 30 cases, by ISH of the EBV EBER-I region and/or immunostaining of latent membrane protein (LMP-I). Of the 16 EBV-positive cases, I 2 also showed IL-10 antigen (AS) in H and RS cells by immunostaining, 5 of the I6 demonstrated hlL-I0 RNA by ISH and 14 of the 16 showed vlL-10 RNA. But only 2 of the 14 EBV-negative cases showed IL-I0 Ag, and one of them showed hlL-I0 RNA, while none demonstrated vlL-I0 RNA. The T cells in the HD-involved tissues were found to be mainly CD4-positive T cells, and had no association with EBV infection. However, the lymphocytes surrounding H and RS cells were more frequently CD4 cells and rarely CD8 cells in the EBV-positive cases, in contrast with the EBV-negative cases. The above results indicate that an EBV infection influenced both cytokine synthesis and the response of T cells in HD.