The aim of this work was to determine the kinetic properties and evaluate the effect of some flavonoids oh human lymphocyte protein tyrosine phosphatase. Tyrosine-, serine- and threonine-phosphate were hydrolyzed by this phosphatase 60%, 20% and 10%, respectively. In the kinetic studies the enzymatic activity was determined by using p-nitrophenylphosphate (pNPP) as substrate. The enzyme presented optimum pH around 5.0 and was inhibited by 100 μM p-chloromercuribenzoate (pCMB) (80%), 10 mM fluoride (35%), 10 mM vanadate (100%), and 5mM Cu +2 (85%). The enzyme was also strongly inhibited by a tyrosine phosphatase inhibitor cocktail, but was unaffected by okadaic acid. These results confirm that the major phosphatase activity in human lymphocytes is a protein tyrosine phosphatase. Among the bioflavonoids tested only fisetin showed an inhibitory effect in order of 80% on the enzymatic activity.273436439Ferreira, C.V., Justu, G.Z., Souza, A.C.S., Queiroz, K.C.S., Zambuzzi, W.F., Aoyama, H., Peppelenbosch, M.P., (2006) Biochem, 88, pp. 1859-1873Jia, Z., (1997) Biochem. Cell Biol, 75, pp. 17-26Cantley, L.C., Auger, K.R., Carpenter, C., Duckworth, B., Graziani, A., Kapeller, R., Soltoff, S., (1991) Cell, 64, pp. 281-302Taylor, S.I., Cama, A., Accili, D., Barbetti, R., Quon, M.J., Sierra, M., Suzuli, Y., Wertheimer, E., (1992) Endocrinol. Review, 13, pp. 566-595Parnetti, L., Senin, U., Meccoci, P., (1997) Drugs, 53, pp. 752-768Zhang, Z.-Y., (2001) Curr. Opin. Chem. Biol, 5, pp. 416-423Casagrande, F., Darbon, J.M., (2001) Biochem. Pharmacol, 61, pp. 1205-1215Aoyama, H., Melo, P.S., Granjeiro, P.A., Haun, M., Ferreira, C.V., (2000) Pharm. Pharmacol. Comm, 6, pp. 331-334Ben-Arie, A., Hagay, Z., Bem-Hurt, H., Open, M., Dgani, R., (1999) Eur. J. Obs. Gyn. Rep. Biol, 86, pp. 69-71