A clonal strain of thyrotrophin (TSH)-dependent rat thyroid cells (FRTL-5) has been used to evaluate the biological activity of reference preparations of both human and bovine TSH. Using the accumulation of intracellular cyclic AMP as a response parameter, the widely used bovine TSH preparation. Armour 'Thytropar', was calibrated against the First International Standard of Thyrotrophin (pituitary TSH), bovine, for immunoassay. Log dose - log response curves were parallel, and a relative potency of 2.4 IU/unit of 'Thytropar' was obtained. Subcultures of FRTL-5 cells were more responsive to both bovine and human TSH than were human thyroid follicular cells maintained as primary monolayer cultures. Dose-response curves for cyclic AMP accumulation were parallel for a single cell type differentially incubated with human TSH (the First International Reference Preparation) and bovine TSH (Armour 'Thytropar') preparations. The relative potencies (units: IU) of bovine-human TSH were of the order of 2.0 when tested on both FRTL-5 cultures and primary human thyroid monolayers. This suggests that in the spectrum of structural differences between TSH receptors of different species, the discriminatory powers of the human and FRTL-5 cell TSH receptor are similar. Thus FRTL-5 cells form the basis of a bioassay system of considerable value in the study of human thyroid stimulators, as we demonstrate in an evaluation of two recent preparations of human TSH.