Relationships between nuclear volume, nuclear DNA content and radiosensitivity were studied on genera of a family, species of a genus, and strains a species. Throughout the materials observed the radiosensitivity was more markedly correlated with the mean DNA content per chromosome than with the estimated interphase chromosome volume. Data involved information about an appreciable reduction in nuclear DNA cdntent in cultivated polyploid plants associated with a proportional reduction in radiosensitivity.