The activity of phenoloxidase in various tissues of Ostrinia furnacalis Guenee fifth-instar larvae parasitized by Macrocentrus cingulum Brischke was investigated. The effects of various physiological factors on the activity of phenoloxidase and melaninization in the hemolymph of O. furnacalis larvae were examined by the injection of calyx fluid, venom, the mixture of calyx fluid plus venom of M. cingulum in vitro. The results showed that the activity of phenoloxidase in hemolymph of O. furnacalis was inhibited significantly by parasitism and the injection of calyx fluid or the mixture of calyx fluid plus venom of M. cingulum (P 0.01), and the production of melanin was also reduced. The percentage of positive FITC-labelled hemocytes of the parasitized host larvae was lower than that of unparasitized larvae (P 0.01). It was concluded that the melaninization could be inhibited by the injected the calyx fluid, venom or other components produced by the parasitic wasp along with laying eggs, in which calyx fluid was probably the main factor inhibiting the ability of immunity of host insect. Colour and luminescence; Colour; Biochemistry; Proteins; Circulatory system; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts Ostrinia furnacalis (Pyralidae); Colour pigments; Melanization; In larval haemolymph, influences of calyx fluid injection & hymenopteran parasite venom; Biochemistry; Calyx fluid; Enzymes; Phenoloxidase activity; In larvae, influences including effects of hymenopteran parasite; Haemolymph; Hymenopteran parasites; Macrocentrus cingulum Macrocentrus cingulum (Braconidae); Toxins and venoms; Lepidopteran hosts; Ostrinia furnacalis; Larval host haemolymph phenoloxidase activity & melanization effect none