Small lymphocytes respond in vitro to virus infections with the synthesis of interferon. But viruses and their nucleic acids are not the only inducer. Recently we have demonstrated that specific antilymphocyte globulins (ALG) (Falcoff et al. 1972a; Falcoff, Oriol & Iscaki, 1972b), and its divalent F(ab′)2 fragments (Falcoff et al. 1972b), are also able to induce interferon synthesis. A number of unrelated non-virus substances share this property: phytohaemagglutinin (Wheelock, 1965), concanavalin (Falcoff, unpublished results), etc.