Partially dissected coleoptiles of barley (Hordeum vulgare L.) in a microculture system were inoculated with Erysiphe graminis (DC) Merat f. sp. hordei Em. Marchal to test for the effects of heat shock and cytochalasin B (CB) on the hypersensitive reaction (HR). The progress of HR, cytoplasmic activity in the host, fungus growth and development of haustoria were directly observed. Heat shock was most effective at 55 °C for 45 s, 30 min before inoculation. With this treatment, cytoplasmic streaming halted immediately but began to recover 13 h after inoculation. HR was completely inhibited; haustorium development and hyphal growth were equivalent to that in a compatible host for at least 2 days. CB inhibited HR at 10 μgml-1 applied continuously starting at 13 or 15h after inoculation. Fungus growth was not inhibited, but cytoplasmic streaming patterns were altered and rates were reduced. These results suggest that HR depends on the movement of host metabolites to the site of the fungus and possibly on the integrity of the cytoskeleton.