none Activation of prophenoloxidase (proPO) in insects is a defense mechanism against intruding microorganisms and parasites. Pattern recognition molecules induce activation of an enzymatic cascade involving serine proteinases, which leads to the conversion of proPO to active phenoloxidase (PO). Phenolic compounds produced by pPO-activation are toxic to invaders. Here, we describe the isolation of a venom protein from the parasitoid, Cotesia rubecula, injected into the host, Pieris rapae, which is homologous to serine proteinase homologs (SPH). The data presented here indicate that the protein interferes with the proteolytic cascade, which under normal circumstances leads to the activation of proPO and melanin formation. Biochemistry; Circulatory system; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Immunology and repair mechanisms Pieris rapae (Pieridae); Haemolymph; Hymenopteran parasites; Cotesia rubecula; Parasite venom protein inhibition of host haemolymph melanization; Immune like reactions; Haemolymph melanization Cotesia rubecula (Braconidae); Toxins and venoms; Vn50 venom protein; Lepidopteran hosts; Pieris rapae; Parasite venom protein inhibition of host haemolymph melanization