Lymphokines play an important role in immune responses to viruses by modulating functions of T lymphocytes. We examined the effects of interleukin-7 (IL-7) on the growth and cytotoxic activity of dengue virus-specific, CD4+ human cytotoxic T lymphocyte (CTL) clones. We then compared the effects of IL-7 with those of interleukin-2 (IL-2). The CD4+CTL clones were stimulated with dengue antigen or monoclonal antibody to CD3 in the presence of IL-7 or IL-2. IL-7 induced the growth of CD4+CTL clones and maintained the antigen-specific cytotoxic activity without inducing antigen-non-specific cytotoxic activity. The levels of the growth and cytotoxic activity induced by IL-7 were similar to those induced by IL-2. IL-7 and T-cell growth factor had an additive effect on the growth of CD4+CTL clones. These results suggest that IL-7 may play an important role in CD4+ T cell immune responses to viruses and that IL-7 is a lymphokine useful for maintaining the growth and antigenspecific cytotoxic activity of CD4+CTL clones in vitro, especially when used with T-cell growth factor.