Interleukin 10 (IL-10), initially designated cytokine synthesis inhibitory factor (CSIF), was originally identified as a product of murine TH2 clones that suppressed the production of cytokines by TH1 clones responding to stimulation by antigen in the presence of monocyte/macrophage antigen-presenting cells. The human homolog of murine IL-10 was subsequently cloned by cross-hybridization. Human and murine IL-10 are 81% and 73% identical at the nucleotide and amino acid levels, respectively. Both mouse and human IL-10 are also highly homologous to a previously uncharacterized open reading frame in the Epstein-Barr virus (EBV) genome, BCRF1. The BCRF1 gene product is now designated viral IL-10. In mouse, IL-10 is produced by TH2 cells, activated fetal thymocytes, macrophages, keratinocytes, and LY-1+ (CD5+) and normal B cells. Cell lines found to have the capacity to produce IL-10 in humans include: TH0, TH1, and TH2-like CD4+ T cell clones; B cell lines derived from patients with AIDS a