A phosphatidyllnosltol(PI) kinase activity associated with certain protein tyrosine kinases important in cell proliferation phosphorylates the 3’hydroxyl position of PI to produce phosphatkfylinosltol-3phosphate(Pl-3-P). Here we report that, in addition to PI-3 kinase activity, anti-phosphotymsine(aP-tyr) immunoprecipitates from platelet-derived growth factor (PDGF)-stimulated smooth muscle cells (SW) contain lipid kinase activities that utilize the substrates phosphatldylinositol-4-phosphate (PI-4-P) and phosphatidylinosltol-4, 5-bisphosphate (PI-4, 5-P2). These activities are absent in aP-tyr immunopreclpltates from quiescent SMC. The product of PM-P phosphorylatlon appears to be phosphatldylinositol-3, 4bisphosphate(PI-3, 4-P2), a lipid not previously reported. The product of PI+-P2 phospholylation Is~ ino! 3itol-mate(PIPa). PI-3-P was detected in quiescent SMC and increased only slightly In response to PDGF. PIP, and the putative PI-3, 4-P2 appeared only after the addition of mitogen. Both the temporal production of these novel phosphollplds after PDGF stimulation and the observation of the enzymatic activities that produce them in aP-tyr immunoprecipltates suggest that these phospholipids are excellent candidates for mediators of the PDGF mitogenic response.