Recently, the mitogenic effects of the Mycoplasma arthritidis supernatant, MAS, and the induction of interferon-gamma (IFN-gamma) and interleukin-6 (IL-6) by MAS have been described. In the present series of experiments we investigated human peripheral blood mononuclear cells (PBM) and human spleen cells with respect to their production of these and other cytokines. In human spleen cell cultures and PBM, MAS induced the synthesis of interleukin-1 alpha (IL-1 alpha) and IL-1 beta. Both interleukins were secreted faster and in higher amounts by PBM. IL-6 was also induced by MAS in PBM and human spleen cells. The amounts of IL-6 measured by ELISA were higher in PBM, whereas the biological activity of IL-6 was higher in spleen cell cultures. T-cell products such as IL-2, IL-4, and IFN-gamma were also induced by MAS in PBM and spleen cells. The kinetics of IFN-gamma and IL-4 induction were negatively correlated. In PBM we found low levels of IL-4 and high IFN-gamma induction, whereas in spleen cells high titers of IL-4 and low IFN-gamma titers were observed. Collectively, our results indicate that MAS induces different networks of cytokine interactions depending on the organ from which the cells are derived.