The CD3(T3)/antigen receptor complex appears to function by transducing an antigen signal presented by macrophages into the hydrolysis of phosphatidylinositol 4, 5-bisphosphate [PtdIns(4, 5)P₂]. In order to find out how the CD3/antigen receptor complex regulates the hydrolysis of Ptdlns(4, 5)P₂ to diacylglycerol and inositol trisphosphate, we investigated the possible role played by a guanine nucleotide-binding regulatory protein in PtdIns(4, 5)P₂ hydrolysis in a human T cell leukemia line, JURKAT. JURKAT cells were made permeable to Al³⁺, F^-, GTP, and a nonhydrolyzable GTP analogue, guanosine 5'-O-(3-thiotriphosphate) (GTPγS), by treatment with pseudomonal cytotoxin. In the presence of AlCl₃ NaF stimulated the release of inositol phosphates in the cytotoxin-treated JURKAT cells. NaF plus AlCl₃ induced increases in inositol tris-, bis-, and mono-phosphates and decreases in PtdIns(4, 5)P₂, phosphatidylinositol 4-phosphate, and phosphatidylinositol within 5min after addition to the cytotoxin-treated cells at 37C. GTPγS stimulated, to some extent, polyphosphoinositide hydrolysis in the cytotoxin-treated JURKAT. The cytotoxin-treated JURKAT cells retained the ability to respond to anti-Leu-4 with polyphosphoinositide hydrolysis. It has been shown that Al³⁺ in the presence of F^- modulates the activity of various guanine nucleotide-binding regulatory proteins. Therefore, the results obtained in this study indicate that a guanine nucleotide-binding regulatory protein regulates the polyphosphoinositide breakdown in JURKAT cells by influencing phosphodiesterase activity.