On the premise that an individual with an intact immune system has the capability to develop both cellular and antibody immune responses supported by the balance between the lymphokines secreted by T helper (Th) cells, we studied the interaction between different types of Th cell clones in vivo and the parameters that may affect this interaction. We used an adoptive transfer system in which nude or lethally irradiated mice were reconstituted with histocompatible CD4⁺ keyhole limpet hemocyanin (KLH)‐specific T cell clones with defined lymphokine profiles. This approach allowed us to study the effects of the cognate interaction between T and B cells in the presence of a defined set of lymphokines. We demonstrated that the co‐transfer of both subsets of Th cells resulted in increased production of IgA, and decreased production of IgE and IgG2a. The concomitant presence of both cell types also increases their functional survival in vivo. We have shown that in the presence of a Th2 clone, higher immunization doses (above 100 μg trinitrophenol (TNP)‐KLH/mouse) result in increased production of IgE and IgG1. In contrast, when a Th1 clone is present, low immunization doses (less than 50 μg TNP‐KLH/mouse) resulted in increased production of IgG2a. We were also able to show that the neutralization of interleukin‐4 (IL‐4) and or interferon‐γ (IFN‐γ) was sufficient to abrogate most of the regulatory effects caused by the Th2 or the Th1 clone respectively. Our results indicate that the subset of T cell(s) transferred determines the type of response obtained. In addition, the data presented indicate that the antigen dose used for immunization can modulate the quantitative parameters of the response. Furthermore, we have shown that the interaction between the two subsets of T cells in vivo is characterized by both antagonistic and agonistic effects and that most of the regulatory effects exerted by one subset over the other are mediated by IL‐4 or IFN‐γ.