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.