To study the role of interleukin (IL)-6 as a growth and differentiation factor for Epstein-Barr virus (EBV)-transformed B lymphocytes, we transfected the cDNA coding for human IL-6 in a monoclonal IgG1-secreting EBV B cell line. Two independent clones were selected that constitutively secreted high amounts of IL-6. These clones showed enhanced levels of IL-6 and tumor necrosis factor alpha secretion when compared to non-IL-6 transfected controls. Moreover, they could efficiently be recovered from low cell density cultures in limiting dilutions when plated on a feeder layer of heterologous EBV B cells. IL-6-induced phenotypical changes comprised a significant rise in immunoglobulin secretion levels and enhanced membrane expression of CD25 (the beta chain of the IL-2 receptor) and of the B cell differentiation antigen CD40. IL-6-dependent down modulation of CD38 and of the adhesion structure VLA4 were also observed. Our data support the notion that IL-6 can serve as an growth and differentiation factor for EBV B cells.