Ecto-5'-nucleotidase (5'-NT,) CD73 is a glycosyl phosphatidylinositol (GPI)-anchored differentiation Ag and purine salvage enzyme expressed on the surface of subsets of human lymphocytes. CD73 mAbs, in combination with submitogenic PMA or OKT3, activate human T cells to proliferate, secrete IL-2, and express IL-2 receptors. For several GPI-anchored proteins implicated in T cell activation, activation is thought to occur through a mechanism that requires the GPI anchor. To investigate the role of the GPI anchor in CD73-mediated signal transduction, CD73 cDNA was transfected into the human T cell leukemia line Jurkat. Transfectants were stimulated to secrete IL-2 by immobilized OKT3 + PMA and by soluble CD73 mAb + PMA. The amount of IL-2 synthesized by individual clones in response to CD73 mAb + PMA was proportional to the IL-2 response to immobilized OKT3 + PMA. CD73 transfectants of Jurkat mutants defective in the TCR, in p56lck, or in the tyrosine phosphatase CD45 did not secrete IL-2 in response to CD73 mAb + PMA, suggesting a role for the CD3/TCR-signaling complex in CD73-mediated signal transduction. A transmembrane form of CD73 was expressed in Jurkat cells by fusing the extracellular portion of CD73 to the transmembrane domain of human tissue factor. Although as a group, clones expressing transmembrane CD73 synthesized less IL-2 in response to CD73 mAb + PMA than clones expressing the GPI-anchored form of the molecule, the responses of the two groups overlapped considerably. In contrast to previous studies with Ly-6, Qa-2, and CD55, the GPI anchor is not critical for activation through CD73.