Tobacco plants (cv. Kyl6) injected into the base of the stem with DL-3-aminobutyric acid or (R)-3-aminobutyric acid were strongly protected against a challenge inoculation with Peronospora tabacina the causative organism of blue mold. The S-enantiomcr of 3-aminobutyric acid, DL-2-aminobutyric acid, or 4-aminobutyric acid induced no protection against the disease. Injections with similar amounts of sodium salicylic acid (SA) or 2,6-dichloroisonicotinic acid (INA) resulted in moderate protection but those made with INA were phytotoxic. Resistance induced by 3-aminobutyric acids was expressed in the upper leaves as minute, chlorotic lesions on which the fungus failed to sporulate. 3-Aminobutyric acids were also effective when introduced as a petiole dip or a soil drench. 3-Aminobutyric acids had no effect on spore germination of P. tabacina nor on its penetration into tobacco leaves. SDS-PAGE analyses revealed enhanced levels of PR-proteins in SA and INA injected plants but not in 3-aminobutyric acids injected plants. On the other hand, PR-protein synthesis was enhanced by all these elicitors when applied as a foliar spray. Ethylene evolution was significantly higher from DL-2-aminobutyric acid-treated leaves as compared to leaves treated with 3-aminobutyric acids. It is proposed that 3-aminobutyric acids protect tobacco against blue mold via a mechanism not related to ethylene production nor PR-protein accumulation. The nature of this mechanism awaits elucidation.