Identification of key factors mediating the differentiation of naive CD4(+) T helper cells into Th1 and Th2 subsets is important for understanding the molecular mechanisms of the development of autoimmune diseases as well as asthma and allergy. Functional importance of a given gene in the initiation of human T helper cell differentiation has been hard to study due to the difficulty in transfecting primary resting human T lymphocytes. In this study we have successfully transfected human primary CD4(+) T helper cells using Amaxa's Nucleofection technology. To overcome the background caused by untransfected cells, we have developed a system for enriching nucleofected unstimulated human primary T helper cells that express the gene of interest. This is achieved by introducing a plasmid construct containing a bicistronic unit coding for a truncated mouse MHC class l H-2K(k) cell surface marker followed by selection of H-2K(k) positive cells using antibody coated beads. We demonstrate that the nucleofected and enriched H-2K(k) positive T helper cells differentiate into Th1 and Th2 cells as well as the non-transfected control cells. We also show that by using this novel method, introduction of an shRNA targeting Stat6, a key molecule driving the Th2 cell development, results in impaired Th2 cell differentiation, as expected. The method described here, enables fast and feasible preparation of highly pure transfected primary CD4(+) T cell cultures ideal for studying the influence of overexpression or knockdown of a given gene on T helper cell differentiation and other primary human T cell functions.