Viral antigen prepared by heat inactivation of vaccinia virus stimulated production of interferon in association with transformation of sensitized human lymphocytes in vitro. Involvement of a macrophage-lymphocyte interaction in production of interferon stimulated by viral antigen was found in which macrophages greatly augmented the amount of interferon produced by lymphocytes. Reimmunization with live vaccinia virus resulted in a selective increase in the ability of lymphocytes to produce interferon in the presence of viral antigen 4-7 weeks later without a corresponding increase in the degree of already significant lymphocyte transformation. There was no correlation between the extent of lymphocyte transformation and the amount of interferon produced. The augmented interferon response after reimmunization described in this study may be a significant component of the protective effect of immunization with vaccinia against disease occurring after exposure to smallpox.Epstein et al. demonstrated that a macrophage-lymphocyte interaction is involved in the mitogen-(1, 2) and antigen-(3, 4) stimulated production of interferon in cultures derived from human peripheral blood. Such interferon production occurred in association with the transformation of small lymphocytes to large blast-like cells observed in response to the nonspecific mitogen, phytohemagglutinin (PHA), and in the more specific immune reaction when sensitized lymphocytes transform in response to purified protein derivative (PPD). In both situations, lymphocytes were identified as the most likely cells responsible for the production of interferon, but the presence of macrophages greatly augmented the amount of interferon produced.Numerous authors have reported the transformation of sensitized human lymphocytes in response to smallpox vaccine (5-1 1). With one exception, the smallpox vaccine used contained live vaccinia virus, and the possibility that replication of vaccinia in leukocytes contributed to the response could not be ruled out. Many other studies have demonstrated interferon production in cultured leukocytes stimulated by vaccinia virus (12, 13) as well as by other viruses (14,15). Furthermore, cultures of leukocytes from animals immune to a given virus produced greater amounts of interferon when exposed to that virus than similar cultures from nonimmune animals (15). The present studies were therefore undertaken to answer the following questions:(a) Can human lymphocytes transform and produce interferon on an immune specific basis in response to a nonreplicating vaccinia virus antigen?Abbreviations: PHA, phytohemagglutinin; VH, heat-treated vaccinia antigen; PPD, purified protein derivative. 2632(b) Is involvement of a macrophage-lymphocyte interaction necessary in the production of interferon as has been observed with mitogen-and nonviral-antigen-stimulated interferon production?(c) Would reimmunization of donors with live vaccinia virus affect the degree of lymphocyte transformation and/or interferon production that occurred in vit...