Interleukin 10, initially designated CSIF, was originally identified as a product of mouse T helper 2 clones that suppressed the production of cytokines by T helper 1 clones responding to stimulation by antigen in the presence of monocyte/macrophage antigen-presenting cells. The human homologue of mouse IL-10 was subsequently cloned by cross-hybridization. Human and mouse IL-10 are 81% and 73% identical at the nucleotide and aa levels, respectively. Both mouse and human IL-10 are also highly homologous to a previously uncharacterized open reading frame in the Epstein-Barr virus genome, BCRF1. The BCRF1 gene product is now designated viral IL-10. In mice, IL-10 is produced by Th2 cells, activated fetal thymocytes, macrophages, keratinocytes, LY-1+ (CD5+), and normal B cells. In humans, Th0-, Th1-, and Th2-like CD4+ T cell clones, B cell lines derived from patients with AIDS and Burkitt's lymphoma, activated monocytes and peripheral blood T cells, including Th2-like CD8+, CD4+CD45RA+ nai