The effects of human recombinant interleukin 1α (20 pg/l-20 μg/l) and 1β (200 pg/l-20 μg/l) on two systems of thyroid cells have been compared. The thyroglobulin and cAMP secretion and the DNA content of human thyroid cells cultured in monolayer and of continuously grown rat thyroid cells, Fischer rat thyroid cell line have been studied. The growth of the rat thyroid cell line was inhibited by interleukin 1β (20 ng/l-20 μg/l), but not by interleukin 1α. None of the cytokines changed the cAMP production of the rat thyroid cells. In contrast, both cAMP production and thyroglobulin secretion were inhibited dose-dependently by the cytokines in human thyroid cells in secondary cultures. These results caution the interpretation and extrapolation of changes induced by interleukin 1 from one cell system to the other.