The cell population necessary for interferon production after mitogen stimulation was investigated in the mouse. Thymus cells and cortisone-resistant thymus cells failed to produce interferon after mitogen stimulation. When spleen cells, a mixture of B and T cells, were treated with anti-K chain serum and complement and then stimulated with PHA, no diminution of interferon production was observed. However, when spleen cells were treated with anti-ϑ antibody and complement and then stimulated with PHA, interferon production was markedly reduced. These studies suggest that peripheralized T cells are required for interferon production after mitogen stimulation.