Cytokines are the most important inducers of T helper (Th) cell differentiation. Interleukin‐12 (IL‐12) and interferon‐α (IFN‐α) are responsible for human Th1‐cell differentiation, while IL‐4 is the critical cytokine promoting Th2‐cell development. These two subsets of cells co‐ordinate immunological responses to pathogens as well as autoimmune or allergic reactions. The pim family of proto‐oncogenes encodes serine/threonine‐specific kinases involved in cytokine‐mediated signalling pathways in haematopoietic cells. Here we demonstrate that expression of pim‐1 and pim‐2 mRNAs is selectively up‐ or down‐regulated in human cord‐blood‐derived CD4⁺ cells freshly induced to polarize towards Th1 or Th2 cells, respectively, whereas their expression is inhibited in both cell types by the immunosuppressive transforming growth factor β (TGF‐β). Moreover, the Th1‐specific cytokines IL‐12 and IFN‐α, but not the Th2‐specific cytokine IL‐4, transiently up‐regulate pim‐1 and pim‐2 mRNA expression in human peripheral blood T cells and natural killer cells. In addition, the Pim‐1 protein levels are strongly up‐regulated by Th1‐specific cytokines in all of these cell types. Taken together, our results suggest that pim genes and their protein products are involved in the early differentiation process of T helper cells.