The efficacy of recombinant interferon-alpha (rIFN alpha). on natural killer (NE;) cell cytotoxic activity, CD3 +, CD4 +, CDS +, CD56 +, HLA - DR + lymphocyte counts, anti-acetylcholine receptor antibody (AChR Ab) levels, single fibre electromyography findings (SFEMG) and clinical course were evaluated in patients with myasthenia gravis (MG). During the IFN alpha treatment (3 mu, subcutaneous, 3 times a week). NK cell cytotoxicity and CD4 -/8 + ratio increased, NK cell count remarkably decreased, and no significant clinical or SFEMG changes were observed. This preliminary open study in MC patients has demonstrated enhanced NK activity per unit NK cell after IFN alpha therapy. Although lymphocyte phenotypes and NK function approached normal levels during therapy, a higher dose of IFN alpha may be required for a significant clinical response. It ha:, been also concluded that 6 months of IFN alpha therapy seems to be safe in MG, though in patients with malignancy. IFN alpha may cause increased autoimmunity, AChR positivity and MG. (C) 1996 Elsevier Science B.V.