Evidence from various systems suggests that thymus-derived lymphocytes can affect the quality of antibody responses by recognizing various portions of the immunoglobulin receptor of bone-marrow-derived thymus-independent lymphocytes. A model for this process is proposed involving two antigen-specific mature T helper cells, one of which also is specific for immunoglobulin determinants. These two cells act synergistically. Evidence from adoptive secondary antibody responses demonstrates that both cells are antigen-specific T cells and that the immunoglobulin-recognizing T helper cell is absent from experimentally agammaglobulinemic mice. This cell is termed an "immunoglobulin-dependent T cell" because its activation requires the presence of immunoglobulin.