Comparative CD and X-ray diffraction studies of DNA compact particules which were obtained in PEG-containing water-salt solutions, have been carried out. Compact particles, formed from native DNA, produce a psi CD spectrum (characterized by a negative band at lambda-270 nm) and a small-angle X-ray diffraction pattern, which shows two reflections: I at 34-40 A and II at 80-90 A (together with its second-order reflection). Compact particules, formed from DNA molecules with partially disordered secondary structure, do not produce the psi CD spectrum and the reflection I, while the reflection II remains unchanged. It is suggested that the spacing of 34-40 A is associated with a side-by-side packing of DNA fragments in microcrystallization' regions in compact particules and that such microcrystallization' accounts for the generation of the psi CD spectrum.