Haemonchus contortus is an economically important gastric parasite infecting sheep and goats. The parasite survives the host immune attack by releasing protective molecules. In the present study, a 55 kDa secretory glycoprotein (gp55) was identified that inhibited host neutrophils as judged by reduced H2O2 production by these cells. The binding of gp55 to neutrophils was confirmed by flow cytometry. This binding was mediated by cellular CD11b/CD18 integrin. The glycoprotein gp55 also bound to goat monocytes and lymphocytes and inhibited monocyte function. Using light fluorescence microscopy, gp55 was localized at the surface of adult worms. The absence of gp55 in the infective L3 larvae and its expression in the blood-feeding stage support a role for gp55 in the parasitic stage of the organism.