Production of Con A-induced helper T cell replacing factor requires a T cell and an Ia-positive non-T cells.

Evidence is presented that indicates that two cells are required for the production of Con A-induced, helper T cell replacing factor. The factor production involves both a T cell and an IA,IE/C-positive non-T cell. The T cell and the Ia-positive cell need not be H-2 compatible. Limiting dilution analyses suggested that the Ia-positive cell is limiting in normal spleen. A hypothesis is put forward that the activation of T cells by Con A may involve "gluing" normally specific, functional T cells to the Ia expressing cells with which they would normally interact in antigen-specific activation.