The invention concerns inhibition of the production of proinflammatory cytokine interleukin-17 (IL-17) by T cells, using an antagonist of interleukin-23 (IL-23). The invention further concerns the use of IL-23 antagonists in the treatment of inflammatory diseases characterized by the presence of elevated levels of IL-17. IL-23 antagonists include, without limitation, antibodies specifically binding to a subunit or IL-17 or an IL-17 receptor. The invention additionally concerns induction of IL-7 production by using an IL-23 agonist.