Delivery of polynucleotides such as plasmid DNA (pDNA). and siRNA to nondividing and primary Cells by nonviral victors presents considerable challenge. In this contribution, we introduce a novel type of PDMAEMA-ba'ied star-shaped nanoparticles that (i) are efficient transfection agents in clinically relevant and difficult-to-transfect human cells (Jurkat T cells, primary T lymphocytes) and (ii) can efficiently deliver siRNA to human primary T lymphocytes resulting to more than 40% silencing of the targeted gene. Transfection efficiencies achieved by the new vectors in serum-free medium are generally high and only slightly reduced in the presence of serum while cytotoxicity and cell Membrane disruptive potential at physiological pH are low. Therefore, these novel agents are expected to be promising carriers for nonviral gene transfer. Moreover, we propose a general design principle for the construction of polycationic nanoparticles capable of delivering nucleic acids to the above-mentioned cells.