alpha-interferon (alpha-IFN) has been used to treat chronic non-A non-B hepatitis in thalassaemic patients with response rates from 45% to 83%. Unfortunately, treatment with alpha-IFN is associated with side-effects which have a negative effect on the quality of life of the patient. Therefore it would be useful if we could distinguish in advance those patients who would benefit from such therapy from those who would not. In the present study we found that the modification of lymphocyte subsets 20 h after the administration of the first dose of alpha-IFN revealed that relative numbers of T helper lymphocytes (CD4+) increased in three non-responding patients and decreased in five responding patients, whereas those of T suppressor lymphocytes (CD8+), and natural killer cells (CD57+, CD16+) decreased in non-responding patients and increased in responding patients. Therefore analysis of the lymphocyte subsets CD4, CD8, CD57 and CD16 before and 20 h after the administration of alpha-IFN can be used to predict the clinical response to treatment with alpha-IFN.