In opiate addicts specific and unspecific immune responses were examined, before and after methadone treatment. Anomalous immune responses were characterized by compromised cellular immunity (functional deficits of polymorphonuclear leukocytes and T-lymphocytes) in association with efficient production of antibodies. After methadone treatment an elevation of leukocyte functions was noted. The presence of elevated titres of circulating immune complexes observed in all the patients tested could bring about a functional exhaustion of neutrophils. The defects of cellular immunity can be considered important risk factors in the pathogenesis of infectious diseases in addicts.