The inhibitory effects of phloridzin dihydrate on the activity of mushroom tyrosinase have been studied. The results show that phloridzin can inhibit the diphenolase activity of the enzyme and the inhibition displays to be reversible. The IC(50) value was estimated as 110microM. The kinetic analysis showed that the inhibition of phloridzin on the diphenolase activity of the enzyme is of competitive type, and the inhibition constant (K(I)) was determined to be 64.3microM. The inhibitory effects of the different concentrations of phloridzin on the monophenolase activity were also studied. There were almost no changes in the lag period and the steady-state rate, while the plateaus in the inhibitory curve lowered with increasing the concentration of phloridzin when using tyrosine as a substrate.