Attempts were made to detect the production of diffusible specific elicitors or suppressors of the hypersensitive reaction (HR) by the lettuce downy mildew fungus Bremia lactucae. In cv. Mildura prior inoculation with the virulent isolate TV did not induce susceptibility to the avirulent isolate IM43. Resistance (HR) was expressed by cells adjacent to compatible intercellular hyphae or even cells containing compatible haustoria or infection structures. Intercellular fluids recovered from cotyledons heavily infected with a virulent isolate of B. lactucae were examined as a potential source of race-specific elicitors of the HR. Irrespective of the isolate/cultivar combination employed, no symptoms developed in lettuce cotyledons injected with intercellular fluids. However, the non-hosts cabbage and broad bean developed macroscopically visible symptoms by 8 days after leaf injection. Intercellular fluids from uninfected tissue caused no necrosis. A sandwich or transplant technique was adopted in order to maximise the exposure of resistant tissues to compounds diffusing from intercellular hyphae of B. lactucae. No necrosis was observed in resistant lettuce cells unless there were signs of penetration and the development of rudimentary haustoria from invading incompatible hyphae. Necrosis was more widespread following transplants to cabbage. The apparent absence of diffusible elicitors or suppressors of race-specific resistance indicates that recognition in lettuce downy mildew disease probably involves intracellular interactions occurring between the plant plasmalemma and the primary vesicle, secondary vesicle or haustorium of the fungus.