The invention is directed to a polypeptide comprising a human IL-2 mutein numbered in accordance with wild-type IL-2 wherein said human IL-2 is substituted at at least one of positions 20, 88 or 126, whereby said mutein preferentially activates T cells over NK cells. D20H and I, N88G, I, and R, in particular have a relative T cell-differential activity much greater than native IL-2, with predicted associated reduced in vivo toxicity. The invention also includes polynucleotides coding for the muteins of the invention, vectors containing the polynucleotides, transformed host cells, pharmaceutical compositions comprising the muteins, and therapeutic methods of treatment.