Deer ked (Lipoptena cervi L.), a haematophagous louse fly of deer, also attacks man and can cause persistent pruritic papules. Nineteen patients with this conditions were examined. They suffered attacks while carrying out their work, or merely walking, in forested areas. The bite papules appeared mostly on the head and back. They were very itchy and resistant to treatment and persisted from 2 weeks up to 12 months. Histologically, a typical insect bite reaction was found without any signs of retained deer ked mouth parts. Direct immunofluorescence showed deposits of C3 in dermal vessel walls in 7 of the 11 papules examined. Skin tests with a deer ked whole body extract were positive in all patients tested, showing both immediate and delayed reactions. Moreover, 57% of the patients tested had elevated serum IgE levels. All these findings suggest that IgE, complement and cell-mediated immune mechanisms are involved in the pathogenesis of deer ked bite reactions.