An in vitro antibody response to bee venom phospholipase A2 (PLA) from peripheral blood mononuclear cells of bee sting-sensitized individuals was achieved after stimulation with PLA and pokeweed mitogen. This stimulation resulted in a secretion of TH1-associated cytokines and induced PLA-specific and nonspecific IgG4 antibody production but not IgE production. The addition of interleukin-4 (IL-4) to this system decreased the secretion of IgG antibodies, whereas secretion of polyspecific IgE was induced. The mitogen was not required if peripheral blood mononuclear cells were enriched with autologous, PLA-specific, resting T-cell clones in the presence of the antigen. In these experiments the cytokine profile of the particular clone determined the antibody class generated. Low ratios of IL-4 to interferon-gamma, induced by the antigen alone or obtained by neutralizing anti-IL-4 antibodies, enhanced IgG4 antibody formation, whereas IgE levels increased at high ratios of IL-4 to interferon-gamma. These results suggest a complementary regulation of the main isotypes, IgE and IgG4, implicated in allergic and protective hyperimmune responses.