Interleukin-6 (IL-6) is a member of the pro-inflammatory cytokine family, induces the expression of a variety of proteins responsible for acute inflammation, and plays an important role in the proliferation and differentiation of cells in humans. Interleukin 6 was originally identified as a T cell-derived lymphokine that induces final maturation of B cells into antibody-producing cells. Recombinant human IL-6 acts on B cells activated with Staphylococcus aureus Cowan I or pokeweed mitogen (PWM) to induce immunoglobulin M (IgM), IgG and IgA production, but not on resting B cells. Anti-IL-6 antibody was found to inhibit PWM-induced Ig production, indicating that IL-6 is one of the essential factors in PWM-induced Ig production. Furthermore, IL-6 was shown to augment the primary and secondary anti-SRBC (sheep red blood cell) antibody production in mice in vivo.