The fine organization of the centromere structural heterochromatin (CSH) in a cell culture of calf trachea (TR) was studied by the methods of light and electron microscopy after fixation in native conditions and after treatments with water Henk's solutions and solutions of Ca2+ of different concentrations. In interphase nuclei the CSH forms compact blocks--the chromocentres, which are connected with the nuclear envelope or the nucleolus. The diameter of the main class of DNP fibers in the CSH, chromosomal arms and chromocentres after fixation of control cells and after treatment with 50% Henk's solution is 20-25 nm. 10-15 nm DNP fibers are largely found in the contacts with kinetochores. After 20% Henk's solution treatment 10-15 nm fibers predominate in the CSH and chromosomal arms. A chromonema--a 100 nm chromatin fiber--is detected in the CSH after treatments with different concentrations of Ca2+ solutions, as well after fixation in native conditions. The peculiarity of structural organization and properties of the calf CSH at interphase and mitosis as compared with analogous regions in mouse chromosomes are suggested to be connected with the composition of its DNA and proteins.