Inhibitory effect of 116 coumarin derivatives (thiosemicarbazides, thiazolidinones, Schiff bases, chalcones, pyrazolines and thiazolidindiones) on polyphenol oxidase activity was investigated. Thirty eight coumarin derivatives inhibited polyphenol oxidase, 26 derivatives showed no effect, and 52 caused apparent increase in enzyme activity. The most effective inhibitors were thiosemicarbazides showing the dependence of the inhibitory effect to the substituent at the position 4 of thiosemicarbazide chain. Among other potential inhibitors 7 thiazolidinones, 7 Schiff bases, 9 chalcones and pyrazolines and 10 thiazolidinediones were found to inhibit polyphenol oxidase. Inhibitory effect of coumarine derivatives was most likely due to interaction between thiosemicarbazide group and copper in the active site of enzyme preventing the access of molecular oxygen, as well as competitive inhibition due to the similarity of the aromatic ring in the coumarin derivatives structure with the substrate L-DOPA. The apparent increase in polyphenol oxidase activity could be attributed to the prooxidative effect of coumarin derivatives on the substrate of polyphenol oxidase L-3 4- dihydroxyphenylalanine and/or to the action of certain coumarin derivatives as polyphenol oxidase substrates. Prooxidative effect of some thiazolidinones, pyrazolins and thiazolidinediones on L-3,4- dihydroxyphenylalanine could be attributed to the oxidation potential of keto groups positioned on thiazo and/or coumarin ring. Coumarin derivatives containing 3,4-hidroxyphenyl group act as substrates of polyphenol oxidase.