Natural killer (NK) cell activity and blood mononuclear cell subpopulations were characterized in patients with Hashimoto's thyroiditis (n = 11), Graves' disease (n = 20), non-toxic goitre (n = 10) and in normal controls (n = 22). NK cell activity against K 562 target cells and the capability of IFN-alpha, Il-2, and indomethacin to enhance NK cell activity in vitro did not differ significantly between the groups. The percentages of large granular lymphocytes, CD5+, CD4+, CD8+ and CD16+ cells were normal in patients with non-toxic goitre, Hashimoto's and Graves' diseases. There was no correlation between NK cell activities and TgAb, MAb and TSAb. Although NK cell activity is suppressed in several autoimmune diseases, NK cell function is normal in patients with autoimmune thyroid disorders.