A novel, potent, and Src family-selective tyrosine kinase inhibitor, PP1, has been identified and characterized. This compound selectively inhibits Lck and FynT, two Src family tyrosine kinases involved in T cell activation. PP1 inhibits anti-CD3-induced tyrosine phosphorylation in T cells and selectively inhibits IL-2 gene expression without affecting the expression of the granulocyte-macrophage colony-stimulating factor (GM-CSF) or IL-2 receptor (IL-2R) genes. It also preferentially inhibits T cell receptor (TcR)-dependent T cell proliferation over non-TcR-dependent proliferation induced by phorbol 12-myristate 13-acetate (PMA) and interleukin-2 (IL-2). PP1 demonstrates selectivity for Lck and FynT over other tyrosine kinases such as ZAP-70, JAK2, and the epidermal growth factor receptor (EGF-R). In vitro studies show that PP1 inhibits Lck and FynT at nanomolar concentrations, while inhibiting other kinases at higher concentrations. PP1 is more effective than other tyrosine kinase inhibitors such as staurosporine and genistein in selectively inhibiting Src family kinases. The compound's ability to selectively inhibit signaling pathways involved in T cell activation suggests its potential as a tool for studying the role of Lck and FynT in T cell signaling. PP1's selectivity and potency make it a valuable new tool for investigating the role of Src family kinases in T cell function.A novel, potent, and Src family-selective tyrosine kinase inhibitor, PP1, has been identified and characterized. This compound selectively inhibits Lck and FynT, two Src family tyrosine kinases involved in T cell activation. PP1 inhibits anti-CD3-induced tyrosine phosphorylation in T cells and selectively inhibits IL-2 gene expression without affecting the expression of the granulocyte-macrophage colony-stimulating factor (GM-CSF) or IL-2 receptor (IL-2R) genes. It also preferentially inhibits T cell receptor (TcR)-dependent T cell proliferation over non-TcR-dependent proliferation induced by phorbol 12-myristate 13-acetate (PMA) and interleukin-2 (IL-2). PP1 demonstrates selectivity for Lck and FynT over other tyrosine kinases such as ZAP-70, JAK2, and the epidermal growth factor receptor (EGF-R). In vitro studies show that PP1 inhibits Lck and FynT at nanomolar concentrations, while inhibiting other kinases at higher concentrations. PP1 is more effective than other tyrosine kinase inhibitors such as staurosporine and genistein in selectively inhibiting Src family kinases. The compound's ability to selectively inhibit signaling pathways involved in T cell activation suggests its potential as a tool for studying the role of Lck and FynT in T cell signaling. PP1's selectivity and potency make it a valuable new tool for investigating the role of Src family kinases in T cell function.