EPR studies pointed out that o-semiquinone free radicals with g-values 2.0038-2.0040 take part in moxifloxacin-melanin complex formation. The process contributed to increase in free radicals concentration in nonirradiated complexes. This effect was observed for the complexes with 1 x 10(-4) M, 1 x 10(-3) M and 4 x 10(-3) M drug concentrations. UV irradiation contributed to decrease in free radicals concentration in DOPA-melanin complexes with moxifloxacin, besides the complexes with the drug concentration of 1 x 10(-4) M. The strongest decrease was observed for DOPA-melanin-moxifloxacin complexes with the drug concentration of 1 x 10(-3) M. Homogeneous broadening of EPR lines, strong dipolar interactions and slow spin-lattice relaxation processes characterized all the tested melanin samples. (C) 2013 Elsevier B.V. All rights reserved.