The absorption and dichroism of muscle glycogen--iodine complexes depending on CaCl2 concentration were studied. It was shown that besides intensification of glycogen staining with iodine, high concentrations of CaCl2 cause destabilization of the alpha-glucan helix as well as disturbances in the formation of a specific chromophore of the iodine-glycogen complex which manifest themselves as a loss of dichroism. The stained chromophore formed by a simultaneous decrease in the dichroic absorption seems to be generated in the non-helical regions of the glycogen molecule and is thus nonspecific. This nonspecific chromophore is a potential source of errors in spetrophotometrical assays of glycogen structure. Study of rabbit skeletal muscle and liver glycogens by the Krisman method based on the use of concentrated solutions of CaCl2 failed to reveal any differences in glycogen structure that are normally detectable at low concentrations of CaCl2. The unfavourable effect of high concentrations of CaCl2 on helix formation should be taken into consideration when studying the stoichiometry of iodine interaction with alpha-glucan.