AbstractAn immediate consequence of T cell activation via the T cell receptor (TcR)/CD3 complex and CD2 antigen is the hydrolysis of phosphatidylinositol‐(4,5)‐bisphosphate and the generation of inositol‐(1,4,5)‐trisphosphate and diacylglycerol which then regulate intracellular calcium and protein kinase C. Changes in cellular levels of phosphoinositides phosphorylated on the D‐4 and D‐5 position during T cell activation have been well documented. Recently it has been proposed that phosphoinositides phosphorylated on the D‐3 position of the inositol ring by a novel phosphoinositide (PI) 3 kinase may also be important in cell activation. In the present study we have examined the levels and regulation of D‐3 phosphoinositides in T cells activated by the TcR/CD3 complex and CD2 antigens. The data show the existence of phosphatidylinositol‐(3)‐monophosphate [PtdIns(3)P], phosphatidylinositol‐(3,4)‐bisphosphate [PtdIns(3,4)P2] and phosphatidylinositol‐(3,4,5)‐trisphosphate [PtdIns(3,4,5)P3] in T cells. Activation of the TcR/CD3 complex or CD2 antigen results in modulation of PtdIns(3,4)P2and a putative PtdIns(3,4,5)P3in T cells but does not change levels of PtdIns(3)P. These data provide the first evidence that lipid products of a P