The plant-derived tumor inhibitors taxodone, taxodione, vernolepin, eupacunin, and euparotin acetate each inhibit the sulfhydryl enzyme, phosphofructokinase. The substrates, fructose-6-phosphate and adenosine triphosphate, protect the enzyme from this, inhibition as does the addition of dithiothreitol to the inhibitors. Incubation of taxodione with phosphofructokinase is associated with the loss of about one sulfhydryl group per inhibitor molecule, and the substrates protect six sulfhydryl groups per protomer of 93,000 daltons.