Study of ultrathin sections of the Chinese hamster normal cells and cells treated with solutions containing decreasing concentrations of Ca++ and Mg++ ions revealed several discrete levels of DNP-fibres organization in mitotic and interphase chromosomes. In total Ca++ and Mg++ concentrations from 4 mM to 0.3 mM, the chromosomes are formed by filamentous elements or chromonema of 100-nm diameter. During gradual decrease of Ca++ concentration to 0.2-0.1 mM the chromosomes decondensate into discrete chromatin structures--chromomeres. In decondensation, the chromomeres acquire a "rosette-like" structure with an electron dense core and DNP-fibers radiating from it. Levels of DNP compactization in chromosomes are supposed to be as follows: the nucleosomal fibres of 10-nm diameter, nucleomeric fibres of 25-nm diameter and chromonema, i.e. a filamentous structure of 100-nm diameter consisting of chromomeres.