Interleukin 1, a product primarily of activated macrophages, was found to exert both a stimulatory and an inhibitory effect on porcine thyroid follicles in suspension cultures. A stimulatory effect on [125I] iodide incorporation into protein (iodination) was seen in cultures exposed to human recombinant interleukin 1 alpha (20 micrograms/l) for 1 h and (0.1 or 10 micrograms/l) for 18 h, whereas an inhibitory effect on iodination of interleukin 1 (10 micrograms/l) was registered after pre-incubation for 42 h and significant upon stimulation with TSH (200 mU/l). Cyclic AMP levels were stimulated in cells exposed to interleukin 1, however, significantly only after 42 h of pre-incubation, whereas TSH-stimulated cAMP response was inhibited by interleukin 1 already after 18 h of pre-incubation. The lumen of interleukin 1 incubated follicles was highly dilated and surrounded by a flattened epithelium. There were, however, no morphological signs of follicle disintegration or cellular cytotoxicity in the cultures exposed to interleukin 1. The data suggest that interleukin 1 might have a modulating role on thyroid function in autoimmune thyroid diseases associated with activation of intrathyroidal macrophages.