Defence properties of hemoctyes were investigated using the anti-hemocyte monoclonal antibody BrH1 obtained by immunizing mice with 2% paraformaldehyde-fixed hemoctyes of the stick insect Bacillus rossius. In Western blot analysis, the antibody recognized a 180 kDa antigen in hemocyte cell lysates, whereas fat body lysates and cell-free hemolymph were negative. In immunofluorescence analysis of cultured or freshly collected hemoctyes, BrH1 stained intracellular antigen(s) in detergent-treated cells. Transverse cryosections of adult stick insects probed by immunofluorescence with BrH1 showed in situ the scattered distribution of hemoctyes inside the haemocoel. The antigen(s) recognized by BrH1 appears to be involved in cell defence hemocyte-mediated mechanisms, as evidenced by the fact that cryosections of insects challenged in vivo with yeast cells, bacteria, or polystyrene latex particles and probed with BrH1 showed an accumulation of antigen surrounding the injected stimuli. Copyright 1997 Elsevier Science Ltd. All rights reserved