Melanins are polymorphous and multifunctional biopolymers with a relatively high concentration of freeradicals. EPR spectroscopy was used to study o-semiquinone free radicals in model eumelanins synthesizedfrom 3,4-dihydroxyphenylalanine (DOPA) and tyrosine in the presence of tyrosinase, and melanins isolatedfrom A-375 and G-361 human melanoma malignum cells exposed to two compounds: 5,7-dimethoxycoumarin(DMC) and valproic acid (VPA). Changes were determined in the concentrations of free radicals in theindividual melanins from tumour cells treated with DMC and VPA. A strong decrease in the concentrationsof free radicals characterizes melanins isolated from tumour cells treated together with DMC and VPA.Slow spin-lattice relaxation processes were noted in the melanins tested with homogeneous broadened EPRspectra. The EPR technique may be useful not only for the elucidation of free radicals in melanins fromA-375 and G-361 cells treated with VPA and DMC but it could also be applied to establish the relationshipbetween melanin type and the malignancy of melanoma malignum