Following treatment with 5-aminolevulinic acid the activity of iS-adenosyl-L- methionineimagnesium protoporphyrin IX methyltransferase (EC 2.1.1.11) in wheat leaves decreased, whereas monomethyl ester magnesium protoporphyrin IX and protochlorophyllide accumulated. The inactivation of the enzyme was the result of a cessation of the enzyme-product complex dissociation because of the limited number of porphyrin-binding sites on the structural carrier.