Paramagnetic complexes of DOPA-melanin with ciprofloxacin, lomefloxacin, norfloxacin and sparfloxacin were studied by EPR spectroscopy. The aim of this work was to determine free radical concentration and properties of these complexes. Free radical concentrations in the studied complexes were similar to 10(19)-10(20) spin/g. Relatively lower and similar (5.1-6.6 x 10(19) spin/g) free radical concentrations characterized DOPA-melanin complexes with ciprofloxacin, and lomefloxacin. Higher concentrations (0.8-1.2 x 10(20) spin/g) were obtained for complexes of norfloxacin and sparfloxacin. Effect of the drug concentration on free radicals in melanin was observed. Strong dipolar spin-spin interactions and slow spin-lattice relaxation processes exist in all of the samples. (C) 2010 Elsevier B.V. All rights reserved.