Fourteen patients with clinically, pharmacologically and electrophysiologically confirmed myasthenia gravis in which therapeutic plasma exchange was performed were analyzed. Condition of these patients was assessed before and after the plasma exchange using the Besinger score. The serum immunoglobulin levels were also determined by ELISA-test before and after the plasma exchange. Findings have shown that clinical improvement was highly significant. Changes in IgM and IgA levels were significant, but not in IG and IgE. Significant correlation neither between the level of each separate immunoglobulin and clinical condition of the patient, nor between the change in the level of each separate immunoglobulin and changed clinical course was found after the series of plasma exchange. Cumulative effect of all immunoglobulins was not observed due to differences in their physical and chemical properties. Absence of significant statistical correlation between the level of each immunoglobulin and clinical condition of the patient as well as between the level changes of each separate immunoglobulin and changed clinical condition after series of plasma exchange could be explained by cumulative effect of all immunoglobulins, but probably also by the presence of some other, circulating factor not belonging to the class of immunoglobulins and associated with the development of myasthenic weakness.