To study the requirements for activation of human Th1 and Th2 cells, soluble peptide/DR1 complexes were prepared from naturally expressed DR1 protein. When immobilized, this material induced T cell activation, as revealed by CD25 up-regulation. Unexpectedly, Th2 cells required a higher density of peptide/DR1 complexes than Th1 cells to initiate CD25 up-regulation. Similar findings were obtained with immobilized or soluble and cross-linked anti-CD3 mAb. In contrast, peptide/DR1 complexes displayed on the surface of nonprofessional APC similarly induced CD25 up-regulation in Th1 and Th2 cells. Signaling events distinguishing human Th1 and Th2 cells following TCR engagement by anti-CD3 mAb were then studied. It was observed that upon TCR triggering, the overall tyrosine phosphorylation profiles were fainter in Th2 than in Th1 clones. Similar results were obtained with Th1- and Th2-polarized polyclonal lines. Varying the dose of anti-CD3 mAb, the kinetics of activation, and coengagement of CD3 and CD28 failed to increase tyrosine phosphorylation in Th2 cells to levels reached in Th1 cells. In contrast, treatment with the tyrosine phosphatase inhibitor phenylarsine oxide resulted in similar tyrosine phosphorylation levels in Th2 and Th1 cells. These findings indicated that Th2 cells had an intrinsically lower TCR-induced tyrosine phosphorylation capacity than Th1 cells, which might be controlled by Th1- and Th2-specific phosphatase profiles. Finally, a weaker association was found between ZAP-70 and CD3zeta in Th2 than in Th1 cells after TCR engagement. Taken together, these results constituted evidence that early events in the TCR signaling cascades are distinct in human Th1 and Th2 cells.