Photophosphorylation by isolated chloroplasts has been reported and studied in several species (21), but no published evidence of the reaction by maize chloroplasts could be found. In a study of this reaction by maize chloroplasts initial experiments using existing isolation techniques (2, 12) failed to give active chloroplasts. One previous report of a smiliar inability to demonstrate photophosphorylation has been made by Heber (11) who was unable to obtain chloroplasts capable of photophosphorylating from Vicia faba. In this case a substance which inhibited the reaction by spinach chloroplasts was found in the bean cell sap. The aims of this study were to determine if maize leaf tissue contained a component that inhibited chloroplast activity and to devise methods of extraction that would permit the isolation of chloroplasts from maize leaf tissue which would be capable of carrying out the Hill reaction and cyclic and noncyclic photophosphorylation.