Although interleukin (IL)-12 was originally purified from an Epstein-Barr (EBV)-transformed B cell line and the high correlation of EBV infection and IL-12 expression has been suggested, no study has reported whether EBV infection is directly linked to IL-12 expression. To address this issue, we have investigated IL-12 expression in B cells during in vitro transformation with EBV. Human peripheral B cells became capable of constitutively producing p40 by in vitro transformation with EBV, coincident with the expression of latent membrane protein 1 (LMP1) of EBV. These B cells expressed p40 and p35 mRNA, and phorbol myristate acetate (PMA) stimulation strongly enhanced p40 and p70 production. Furthermore, transfection with LMP1 expression vector into a human B lymphoma cell line, Daudi, led to p40 production with nuclear factor (NF)-kappaB activation. These results suggest that transformation of primary B cells with EBV induces IL-12 expression potentially through LMP1 expression.