Plant chromosome DNAs released from intact nuclei were analysed in PFGE and it showed that rice chromosome DNAs existed in a form of "240kb-like units" in a releasing condition of this experiments. These units with restriction enzyme digestion were shown to be chromosome-size DNAs up to 1500kb. The possibility of these "240kb-like units" as fundamental organization units of chromosome DNAs was discussed. This technique has been established for a basis of isolating and purifying large DNA fragments, constructing YAC library and restriction enzyme analyzing.