Flower-inducing activity of lysine was examined in Lemna paucicostata 151, a weakly responsive short-day plant, cultured on nitrogen-rich medium under long-day conditions (continuous light). Lemna paucicostata 151 was homogenized in a solution of lysine and the homogenate was centrifuged. The supernatant (lysine-containing extract) was added to nitrogen-rich medium after passage through a membrane filter to give various concentrations of lysine in the medium. Flowering was induced in plants grown for six days on medium that contained lysine at concentrations above 0.25 muM. In plants grown on medium that contained 1 muM lysine, a significant flowering response was observed on the fourth day of culture. However, the flower-inducing activity of lysine disappeared when the lysine-containing extract was added to the medium and the medium was then autoclaved, suggesting that the active principle is unstable to autoclaving. Among derivatives of lysine tested, lysine hydroxamate had the highest flower-inducing activity and lysyl lysine had almost same activity as that of lysine. When added to the medium without homogenization with plant material, lysine and lysyl lysine had flower-inducing activity but lysine hydroxamate did not induce flowering.