The effector memory CD4+ Th17 cells play critical roles in bacterial immunity and pathological inflammation in autoimmune conditions. ZSWIM1 is a gene encoding a protein of unknown function in leukocytes-but containing a zinc finger SWIM motif. In peripheral blood mononuclear cells, the expression of ZSWIM1 is highest in lymphocytes, and in particular shows greatest abundance in naive CD4+ T cells. Upon polarisation of naive CD4+ T cells, ZSWIM1 expression is retained in Th17 cells but is selectively down regulated in Th1 cells. Similarly in in vitro expanded effector memory CD4+ T cells, ZSWIM1 was more abundant in Th17 cells compared to Th1 or Th17 polyfunctional (Th17pf) cells, which produce IL-17A and IFN?. Although stimulation of cytokine production by PMA and ionomycin reduced ZSWIM1 expression, the relative differences in abundance between the cell types were maintained. The activation sensitive nature of ZSWIM1 expression suggests that it may play a novel role in the development or function of T helper cells.