Susceptibility toHelminthosporium sativumin barley coleoptiles is associated with tissue senescence. Aqueous extracts of young coleoptiles show high antifungal activity. This activity disappears rapidly with age owing to the appearance of an inhibitor in the extracts. Preliminary characterization of the antifungal factor suggests an empirical formula of C9H17O7N. Inhibition of activity was found to be caused by various divalent cations. The results point to the particular importance of calcium.