Two forms of mitochondrial creatine kinase (Mi-CK) having Mr 320 kDa and 240 kDa as determined by gel-filtration on Sephacryl S-300 in 0.1 M Tris-HCl pH 7.4 were investigated. The sedimentation coefficient values for these two forms were found to be identical and equal to 12.3 S. When studied by electron microscopy the main type of images for the 320 kDa and 240 kDa Mi-CK appeared as annular particles, 12-14 nm in diameter, with a well-detected subunit structure and a central hollow, 3-4nm in diameter filled with the dye. The results of the averaging of the main type of individual Mi-CK images and particles of the two-dimensional crystal layer point to the overall geometry of the Mi-CK molecule structure as containing eight subunits arranged by a 4-fold symmetry around the central hollow. It may be that the eight identical subunits of crystalline Mi-CK are arranged with a P422 symmetry. However in both cases the averaged main images do not show a mirror symmetry. The multiplicity of the observed projections close to annular one provides additional evidence in favour of the great lability and structural mobility of the Mi-CK subunits. It allows to assume that two forms (320 kDa and 240 kDa) are not the different oligomers but they are two functionally distinct conformational states of octameric molecule of Mi-CK.