The major route of ammonia assimilation is the reaction which is catalyzed by glutamine synthetase to give ammonia. Cell-free extracts and purified thylakoid membranes using differential centrifugation and density gradient techniques were assayed for the percentage activity of the enzyme. Glutamine synthetase was detected in all cell-free extracts. Seventy six percent of the enzyme activity was found associated with the thylakoid membranes. Using antiserum raised to the thylakoids, 78.5% inhibition of the enzyme activity was obtained.