Aged mice preferentially lose the capacity to make IgG and high affinity PFC after immunization with the T-dependent antigen DNP-BGG. We have found that thymectomy accelerates the appearances of these immune deficiencies associated with aging. When splenocytes from old mice are transferred to young lethally irradiated, syngeneic mice and the recipients immunized 7 wk later, the number of IgG and high affinity PFC was increased compared to the response of old splenocytes transferred to young thymectomized mice. These immune deficiencies of aged mice were also reversed when old mice were treated with thymopoietin in vivo or splenocytes from old mice were incubated with thymopoietin before adoptive transfer to young irradiated, thymectomized syngeneic mice. The T-cell independent response to DNP-Ficoll was less impaired than the T-cell dependent response to DNP-BGG in old animals. These data suggest that a decline in thymic function that occurs during aging may contribute to the immunological deficiencies of old animals.