The hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) has been shown to be enhanced in cells transformed by some types of oncogenes and tumor viruses, but it is still unknown whether the breakdown of PIP2 plays a role in oncogene-induced cell proliferation. For examination of this problem, monoclonal antibody specifically directed to PIP2 was injected into cells. This antibody bound to endogenous PIP2 and so inhibited its intracellular breakdown. Injection of this antibody into ras-transformed cells cultured in the presence of serum caused reversible and dose-dependent decrease in proliferation and reversion of the cell morphology to that of the normal phenotype. The antibody also inhibited the proliferation of src- and erbB-transformed cells but had no effect on the proliferation of untransformed or myc-transformed cells. These results show that the breakdown of PIP2 is involved in the signaling pathways for mitogenesis in cells transformed by oncogenes such as ras, src, and erbB.