Korean-style fermented soybean paste (KFSP), Doenjang, is a traditional food that is consumed as a protein source in Korea. Recently, efforts to identify biolgocial response modifiers (BRMs) have been focused on food products. Accordingly, this study which isolated abiologically active substance form KFSP, named KFSP-BRM, ws defined to be aheat-stable carbohydrate with a molecular weight of 2,000 kDa. The biological activity of KFSP-BRM was not inactivated by treatment with an anti-LPS antibody. The oral as well as intraperitoneal treatment of mice with KFSP-BRM significantly enhanced the number of B cells expressing surface significantly enhanced the number of B cells expressing surface immunoglobulins (IgM and IgG). Subsequently, an increased level of immunoglobulins in the sera was also observed. In vitro. KFSP-BRM was found to upregulate the production of interleukin-1 (IL-1) and IL-6 by mactro phages and B cells but not the production of IL-2 by T cells. In conclusion, these data demonstrate the presence of a BRM in KFSP, which may provide an additional benefit to those consuming it is a food. KFSP-BRM is a novel B cellmitogen distinct from fresh soybean lectin or B cell mitogens, such as LPS and Streptococcus protein A. The major biological effects of KFSP-BRM would appear to be anincreased production of IL-1 and IL-6 by macrophages and B cells, thereby enhancing the function of mature B cells.