The Leu 23 Ag is a phosphorylated 28 to 32-kDa disulfide-linked homodimer expressed on the surface of activated T cells, B cells, and NK cells. Resting, unstimulated peripheral blood T cells are Leu 23-, but 20 to 30% of normal thymocytes constitutively express Leu 23. Triggering of the CD3/TCR complex by mAb induced the expression of Leu 23 within 2 h after stimulation. Leu 23 was still detectable on the cell surface 48 h after the stimulus was removed. The induction of Leu 23 strictly correlated with the extent of CD3/TCR cross-linking on the surface of T cells. Anti-CD3-coupled beads, but not soluble IgG or IgM mAb, were able to induce Leu 23 expression on peripheral blood T cells. Soluble anti-CD3 mAb were sufficient to induce Leu 23 on the Jurkat T cell line, but a direct relationship was found between CD3 cross-linking valency and Leu 23 expression. RNA and protein synthesis were both required for Leu 23 induction. Moreover, only CD3/TCR-mediated signals able to stimulate PKC and maintain elevated intracellular calcium ion concentration for greater than 60 min resulted in Leu 23 induction. These findings suggest that the extent of CD3/TCR cross-linking influences the persistence of elevated intracellular calcium ion concentration and PKC activation, and that this in turn determines the commitment of the T cell to activate gene transcription programs necessary for Leu 23 expression. Induction of Leu 23 represented the first detectable cell surface marker after triggering via the CD3/TCR and thus provides an ideal indicator to monitor the very early events in T cell activation.