Phytohemag-glutinin (PHA), poke weed mitogen, and streptolysin-O, all agents capable of stimulating mitosis in human lymphocytes in vitro also stimulated the production of an antiviral substance with biological, chemical, and physical properties identical to those of human interferon. Upon removal of PHA from human lymphocyte cultures interferon production ceased within 2 hr. Readdition of PHA resulted in reinitiation of interferon production.