The proportion of the lymphocytes which produce the cytokines corresponding to murine T helper- (Th1) or Th2 cells was studied using flow cytometry in systemic lupus erythematosus (SLE). When the peripheral mononuclear cells were stimulated with phorbol myristate acetate and ionomycin in the presence of monensin, which blocks the secretion of cytokines, the positive rates for the cytoplasmic IL-2 and IFN-gamma were lower and those for the cytoplasmic IL-4 and IL-10 were higher in SLE than in normal subjects. When the cells were cultured with monensin alone, the positive rates for these 4 cytokines were slightly increased in SLE. These data suggest that the mononuclear cells are already activated in vivo and a deviation of the proportion of the Th cells to the Th2-like ones might be associated with the polyclonal B cell activation seen in SLE.