Methionine (MET) is considered the first committed precursor of ethylene (C2H4), and the pathway has been established as MET → S-adenosylmethionine (SAM) → 1-aminocyclopropane-1-carboxylic acid (ACC) → C2H4. It has been suggested that another pathway to C2H4 may exist, and this pathway has been labeled System II. Our objective was to evaluate several compounds as possible precursors of System II C2H4 production. `White Sim' carnations were placed continuously in 20 mM solutions of MET, ACC, δ-aminolevulinic acid, glutamic acid, α-ketoglutarate, or homocysteine. Deionized water was the control. C2H4 production from the entire flower was measured, and ACC in the basal portion of the petals was quantified. Flowers treated with ACC exhibited the greatest C2H4 production and accumulation of ACC. Homocysteine caused greater production of C2H4 and accumulated more ACC than MET and the other possible precursors. These results suggest that homocysteine may be involved in System II C2H4 production in senescing carnation petals.