The rat thyroid cell strain FRTL-5 was used to investigate the relationship between cyclic AMP and iodide accumulation responses to thyroid-stimulating immunoglobulins (TSIg). Immunoglobulin G-enriched precipitates of sera from 19 of 21 (90%) newly-diagnosed Graves' disease patients gave significant (P less than 0.01) accumulation of iodide (125I), and 16 of these also stimulated intracellular cyclic AMP. Correlation was poor however, with certain TSIg preparations giving widely divergent responses. After initiation of antithyroid treatment, 40% of sera investigated contained TSIg detectable in both bioassay systems, and all but one of the remainder were stimulatory in one of the two bioassays. All patients in remission were devoid of detectable TSIg as determined by iodide accumulation, although a single preparation stimulated cyclic AMP accumulation. LATS-B, a lyophilized reference serum preparation containing high TSIg activity, enhanced iodide accumulation, which showed evidence of correlation with intracellular cyclic AMP at doses above 0.5 mU/ml. At lower doses, iodide accumulation was observed in the absence of detectable cyclic AMP accumulation. TSH and LATS-B-induced iodide accumulation were enhanced, and iodide efflux reduced, by the anion channel blocker 4-4' diisothiocyanate stilbene 2,2' disulphonic acid (DIDS). In contrast, Ig-enriched fractions of normal sera decreased both basal and stimulated iodide accumulation, but were without effect on efflux. TSIg from untreated Graves' sera gave widely-differing iodide accumulation responses which showed poor correlation with both intracellular cyclic AMP and cyclic-AMP-independent iodide efflux. This clear dissociation of responses to serum Ig preparations suggests that iodide uptake in FRTL-5 cells, which do not organify iodide, may be subject to variable effects of non-TSIg components of Graves' sera, on both iodide uptake itself, and as inhibitors of TSIg-induced accumulation of intracellular cyclic AMP.