IgA immunoglobulin fractions containing antibodies to Epstein‐Barr virus (EBV)‐induced membrane antigens were isolated from the sera of two patients with nasopharyngeal carcinoma (NPC), from one non‐NPC patient, and from three persons with sera negative for IgA antibodies. IgA antibodies were not cytotoxic against cells expressing EBV‐induced membrane antigens in the antibody‐dependent cellular cytotoxicity (ADCC) assay. However, IgA antibodies blocked IgG‐mediated ADCC, which indicated that these antibodies could serve as a blocking “factor” in patients with disease and, therefore, were potentially detrimental to the host.