The present study used cocultures of clonally derived B and T cells, together with an antigen reactive with the membrane Ig of the clonal B cells, to address the issue of B-cell differentiation requirements. The B cells were CH12.LX, an in vitro grown subclone of a murine B-cell lymphoma, which bears surface IgM reactive with sheep erythrocytes. The T cells were alloreactive T-helper-cell hybridomas. Very small numbers of T-helper cells could induce differentiation of cloned B cells without the presence of accessory cells, but such induction was dependent upon the presence of the antigen recognized by the B cell. Induced differentiation of the B cells did not depend on metabolic activity of the T cells, and metabolically active T cells could be replaced by fixed cells or by monoclonal antibody reactive with the class II molecule of the B cell to deliver an Ia-specific differentiative signal. T cells, or alloantibody that reacted with the I-E molecule, induced differentiation of the B cells; those that reacted with the I-A molecule did not. These results define the minimal requirements for major histocompatibility complex-restricted, T-cell-mediated induction of B-cell differentiation.