Potato tuberization is induced under short days but substance which triggers it is still unknown. Tuber-inducing activity was detected in leaves and old tubers using single-node stem segment culture in vitro as a bioassay method. The activity in leaves increased under short days, but remained almost constant under long days. It was also found in physiologically old tubers. Anion exchange chromatography of the ethanol extract gave an eluate that exhibited strong tuber-inducing activity. Partial purification revealed the presence of two kinds of active substances, one of which seemed to be a glycoside of the other. These substances appear to be different from known plant hormones.