IL-9 is a pleiotropic lymphokine, one of its activities being the growth stimulation of certain CD4+ T lymphocytes. In murine cutaneous leishmaniasis, depending on the genetic background of the host mouse strain, vigorous proliferation of either mainly Th1 in resistant C57BL/6 mice or Th2-type CD4+ T cells in susceptible BALB/c mice occurs after infection with Leishmania major (L. major). Since little is known about the involvement of IL-9, the possible role of this cytokine with regard to its immunregulatory function was evaluated by comparing its presence in the serum and its expression kinetics in spleen and lymph nodes in resistant and susceptible mice. To this sera of L. major-infected mice were tested functionally for IL-9. In addition the PCR-aided detection of IL-9 mRNA in organs of mice and measurement of the lymphokine in supernatants of restimulated lymph node and spleen cell cultures were used. We show here that although no functionally active IL-9 was detected in sera of both BALB/c and C57BL/6 mice, IL-9 is produced after in vitro antigenic restimulation and its mRNA was found to be expressed in lymph nodes and spleens during an immune response against L. major. Shortly after infection no principal differences in the kinetics of IL-9 expression could be observed, which had its maximum between day 5 and 7 after infection. The rate of production however was higher in the susceptible BALB/c mice. In athymic BALB/c nu/nu mice and in mice depleted of CD4+ T cells no IL-9 production was detectable in vivo at the level of mRNA and no IL-9 was produced after stimulation with L. major antigen in vitro. Treatment of infected mice with cyclosporin A ablates antigen-specific IL-9 production when tested in vitro without affecting its production after polyclonal T cell stimulation. Positively selected, purified CD4+ T cells were fully capable of producing IL-9. From 4 weeks after infection, IL-9 synthesis was observed only in BALB/c mice, correlating with the expansion of antigen-specific Th2 type T helper cells in these mice. Treatment of BALB/c mice with neutralizing anti-IL-4 mAb, a regimen known to lead to subsequent cure of infected BALB/c mice, suppressed late IL-9 synthesis.