Interleukin-4 (IL-4) is known as an immunomodulatory cytokine secreted by relatively few cell types, for example, activated T lymphocytes, basophils, and mast cells, but not by B cells. It plays an important role in promoting the production of the IgE antibody. We established Epstein-Barr virus (EBV)-positive B cell lines from peripheral blood mononuclear cells of patients with atopic dermatitis (AD) and tested the production of several cytokines in the cell lines. We found that IL-4 was produced in a cell line, OB, by an IL-4-specific enzyme-linked immunosorbent assay. IL-4 mRNA was detected in OB and two other AD-derived cell lines by IL-4-specific reverse transcription-polymerase chain reaction. IgE was also produced by the OB cells. The production of IL-4 and IgE was enhanced in the cells treated with 12-O-tetradecanoyl phorbol-13-acetate. This is the first evidence that IL-4 is produced by an EBV-transformed B cell line.