Cellular (erythrocytic) and extracellular (blood serum) calcium levels have been measured in patients with Reiter's disease and with urogenital chlamydiosis over the course of treatment. Calcium metabolism disorders depending on the disease severity have been detected in chlamydiosis patients. Calcium homeostasis imbalance manifests as a significant rise of the red cell calcium level in the presence of normal Ca ion concentration in the blood serum, which fact may result from impaired Ca ion transport through the plasma membrane. Combined therapy, including erythromycin, resulted, besides Chlamydia elimination, in normalization of Ca metabolism parameters under study.