This study investigates the activity of a novel protein-tyrosine kinase inhibitor, PP1, which is selective for the Src family of tyrosine kinases. The compound was found to inhibit Lck and FynT, two Src family kinases crucial for T cell receptor (TCR)-dependent T cell activation. PP1 demonstrated nanomolar potency in inhibiting Lck and FynT, and it preferentially inhibited anti-CD3-induced protein tyrosine phosphorylation and T cell proliferation compared to other Src family kinases such as ZAP-70, JAK2, and EGF-R kinase. Notably, PP1 selectively inhibited the induction of the IL-2 gene without affecting the expression of other cytokine genes like GM-CSF or IL-2R. These findings suggest that PP1 is a powerful tool for studying the role of Lck and FynT in T cell signaling and activation.This study investigates the activity of a novel protein-tyrosine kinase inhibitor, PP1, which is selective for the Src family of tyrosine kinases. The compound was found to inhibit Lck and FynT, two Src family kinases crucial for T cell receptor (TCR)-dependent T cell activation. PP1 demonstrated nanomolar potency in inhibiting Lck and FynT, and it preferentially inhibited anti-CD3-induced protein tyrosine phosphorylation and T cell proliferation compared to other Src family kinases such as ZAP-70, JAK2, and EGF-R kinase. Notably, PP1 selectively inhibited the induction of the IL-2 gene without affecting the expression of other cytokine genes like GM-CSF or IL-2R. These findings suggest that PP1 is a powerful tool for studying the role of Lck and FynT in T cell signaling and activation.