Fungal culture filtrates, cytoplasm, and cell walls of Ophiostoma ulmi contain molecules that elicit mansonone (phytoalexin) accumulation in elm calli. The elicitors from nonaggressive isolate Q311 cause a more rapid reaction on bioassays than those from the aggressive isolate MH75. On the callus of a susceptible elm (Ulmus americana), the elicitors from both isolates cause higher amounts of mansonone production than on the callus of a resistant elm (Ulmus pumila). These elicitors are heat stable, and their eliciting activities are partially sensitive to NaIO4, pronase E, proteinase K, and various hydrolytic enzymes. However, β-glucosidase treated elicitors have increased eliciting activities. The binding of Q311 culture filtrate elicitors to a concanavalin A – sepharose 4B column suggests that glycoproteins may be involved in elicitation. Some abiotic factors stimulate only small amounts of mansonone F accumulation in treated elm cells.