There are several isoforms of interleu kin (IL) ‐15 generated by alternating splicing. We reported previously that alternative IL‐15 transgenic (Tg) mice expressing an IL‐15 cDNA isoform encoding nonsecretable IL‐15 protein had an impaired ability to produce IL‐15. In this study, we found that expression of endogenous IL‐15 mRNA but not tumor necrosis factor a mRNA was severely impaired in response to lipopolysaccharide, not only in macrophages from al ternative IL‐15 Tg mice but also in RAW264.7 cells that had been transfected with alternative IL‐15 together with IL‐15 receptor α (IL‐15Rα). IL‐15 promoter activ ity was suppressed in the transfected cells. Although nuclear factor‐kB activation was not impaired, the binding activity of nuclear extracts to the interferon‐ stimulated response element of the IL‐15 promoter region was reduced in RAW264.7 cells, which had been cotransfected with alternative IL‐15 and IL‐15Rα. IL‐15 was mainly colocalized with IL‐15Rα at the cytoplasmic membrane of RAW264.7 cells, which had been cotrans fected with normal IL‐15, whereas nonsecretable IL‐15 was colocalized with IL‐15Rα in nucleus after cotrans fection with alternative IL‐15 and IL‐15Rα. These re sults suggest that nonsecretable IL‐15 generated by alternative splicing suppresses further IL‐15 gene tran scription, implying a novel autocrine regulatory mech anism for cytokine gene expression by alternative splicing.