Alcohol extraction was made from seedlings of U. americana L. first inoculated with either a nonaggressive or an aggressive strain of O. ulmi (Buisman) Nannf. and cross-inoculated 2 wk later. The content of mansonones A, C, E, F and G, phytoalexins in U. americana, was determined weekly for 1-6 wk after the 1st inoculation. Inoculation with the nonaggressive strain led to greater levels of mansonones than did inoculation with the aggressive strain. The aggressive strain counteracted the effect of the nonaggressive strain in cross-inoculated seedlings. Mansonones F and E were the 2 quantitatively most important mansonones, representing 40.0 and 32.4%, respectively, whereas mansonones G, C and A represented 15.2, 8.7, and 3.7%, respectively, of the total quantities of mansonones, A, C, E, F and G present in each sample.