Signal transduction activity was markedly elevated in cancer cells as shown by the increased activity of enzymes utilizing 1-phosphatidylinositol, PI (PI 4-kinase and PI-4-phosphate 5-kinase) for the production of the second messenger inositol 1,4,5-trisphosphate, IP3, in rat hepatomas (Cancer Res. 54: 2611;5574, 1994) and in human ovarian and breast carcinoma cells (Life Sci. 55:1487, 1994). Quercetin, a flavonoid, in human breast carcinoma MDA-MB-435 cells produced growth inhibition (IC50 = 55 microM) and cytotoxicity (LC50 = 26 microM). Quercetin inhibited PI kinase activity in extracts of breast carcinoma cells (IC50 = 6 microM) and in cultured cells (IC50 = 10 microM) with a minor inhibition of PIP kinase activity. IP3 concentration decreased in parallel with PI kinase activity. In time sequence studies quercetin in breast carcinoma cells brought down PI kinase and IP3 concentration in 60 min to 5 and 6%, respectively; PIP kinase activity was at 63% of controls. The results demonstrate for the first time in proliferating human breast carcinoma cells a reduction by quercetin of the increased capacity for signal transduction, thus providing a novel and sensitive target in cancer cells.