The ability of a substance to block specifically the action of a plant hormone in the intact plant to the extent reported here is unparalleled in plant biology. The most outstanding antiethylene properties of Ag(I) are its persistence, specificity, and its lack of phytotoxicity at effective concentrations. In addition to the responses reported here, Ag(I) has also been found to block ethylene action in tomato and cucumber plants. Although the basis for this protection by Ag(I) is unknown, it is not due to a general scavenging for ethylene by Ag(I), since the concentration of ethylene entering and leaving the chamber was always the same. Furthermore, Ag(I) does not irreversibly bind ethylene, and is a much less effective trap for ethylene than Hg(II), for example, which has only a marginal effect in blocking ethylene action. In contrast to the weak ethylene antagonist, CO2 (1), which shows classical competitive inhibition kinetics by Lineweaver-Burk-type plots, Ag(I) shows noncompetitive behavior (E. M. Beyer, Jr. unpublished data).