The formation of reactive oxygen metabolites in haemolymph of Galleria mellonella larvae Was Studied by ESR spectroscopy. The inhibition of the production of the reactive oxygen metabolites of DOPA in haemolymph under the action Of fungal infection was shown using spin trap 1-hydroxy-3-carboxy-pyrrolidine. This inhibition correlated with decrease of phenoloxidase activity in haemolymph of infected insects. Simultaneously, the decrease of production of DOPA-semiquinone was detected using method of spin stabilization by diamagnetic metal ions. Moreover, it was shown that the formation of DOPA-quinone was slowed down in haemolymph of infected insects. Our results suggest that the DOPA-derived quinones/semiquinones may be involved in immune response of insects as part of its defense mechanism. (C) 2002 Elsevier Science (USA). All rights reserved.