Zinniol is a non-host selective phytotoxin which was originally isolated from culture filtrates of Alternaria zinnia, and then from those of A. dauci. This toxin induces several symptoms on zinnia and carrot and also inhibits seed germination of various plants. Zinniol has been shown to produce necrosis on leaves of susceptible marigold (Tagetes erecta) plants, probably through calcium regulation. The phytotoxin binds to carrot protoplasts and isolated membranes in a saturable and reversible manner. Receptor occupancy stimulates the entry of calcium into protoplasts. Zinniol can partially reverse the effects and binding of the calcium-channel blockers Desmethoxy-verapamil and bepridil. Selected cell lines that are insensitive to Zinniol lose part of their binding capacity and sensitivity to the action of the agonist-like compound but are still able to bind calcium-channel blockers.