Both B lymphocytes and cytotoxic T lymphocytes respond to signals from the T helper (Th) compartment, and such signals are mediated by a number of biochemically distinct factors. This raises the question whether help for B cells and T cells is a function of one or several different kinds of Th cell. Here we describe an in vitro and in vivo study of this problem, using a Th clone, designated MTH-1. The clone carries the cell surface markers Thy-1 and L3T4a, but lacks Lyt-2. It recognizes a minor alloantigen shared by DBA/2, B10.D2 and NZB spleen cells, and such recognition is restricted by H-2Ed. Recognition of antigen in vitro is accompanied by secretion of IL-2. In vivo, both primary and secondary CTL responses to multiple minor alloantigens are enhanced by small numbers (less than or equal to 10(4] of MTH-1 cells. Recognition of alloantigen in a T-depleted B cell population results in the polyclonal activation and maturation of the B cells to secrete immunoglobulin; also, antigen-primed B cells are augmented in their in vivo synthesis of specific antibody to the Thy-1 X 1 alloantigen by around 10(5) MTH-1 cells. Taken together, these results suggest a single Th clone can help both B cells and T cells.