Requirements for an in vitro secondary antibody response to the soluble antigens dinitrophenylated keyhole limpet hemocyanin and dinitrophenylated fowl gamma‐globulin are described for the clawed toad, Xenopus. Priming of both hapten and carrier‐specific cells is required in order to obtain good responses to the hapten. The carrier‐reactive (“helper”) cells do not adhere to nylon wool, are X ray‐resistant and surface Ig‐negative. The cell adherence and X ray sensitivity characteristics of these Xenopus “T” cells and their ability to generate a mixed lymphocyte response, allogeneic cytotoxic and helper activities, are identical to those of thymus‐derived (T) cells in higher vertebrates. The hapten‐reactive, antibody‐secreting cell precursors are nylon‐adherent, X ray‐sensitive and surface Ig‐positive. In the absence of other distinguishing serological markers for T cells in frogs, and considering the thymus dependency of the helper activity described here, the evidence presented is interpreted as demonstrating the requirement for antigen‐specific T‐B cell cooperation in response to soluble protein antigens in vitro for Xenopus.