Therapeutic preparations of FSH, used primarily for treatment of infertility, are calibrated by in vivo bioassay against international standards (IS) derived from different sources deemed appropriate to their use according to pharmacopoeial monographs. Menotrophins, which have been used for several decades to treat infertility, have been calibrated against the IS for urinary FSH and LH (ISU) but are now being replaced by highly purified urinary FSH or rDNA-derived FSH (rFSH). The aim of this study was to evaluate two preparations of human rFSH and one preparation of highly purified urinary FSH as candidate WHO IS for bioassay in an international collaborative study by 27 laboratories in 12 countries, and to characterise them in a range of in vitro bioassays and immunoassays. The biological activity of the three candidate standards was confirmed by all laboratories using all assays contributed to the study. Dose-response relationships by in vivo bioassay for any of the candidate standards did not differ significantly from that for the ISU. Dose-response relationships obtained in in vitro bioassays and immunoassays were also broadly similar among these preparations although dose-response lines for some preparations appeared to be non-parallel in some immunoassays. For each of the three candidate IS, estimates of the relative potency in terms of ISU by in vivo bioassay did not differ significantly between laboratories. In contrast estimates by immunoassays and in vitro bioassays showed significant differences between laboratories. Estimates of relative potency of the highly purified candidate IS materials in terms of one another exhibited less inter-laboratory variability than estimates in term of ISU. Each of the candidate standards showed adequate stability to serve as an IS. On the basis of the results of this study rFSH (code 92/642) was established as the first IS for FSH, human, recombinant for bioassay with an assigned unitage of 138 IU per ampoule and urinary FSH (code 92/512) was established as the first IS for FSH, human, urinary (urofollitropin) for bioassay with an assigned unitage of 121 IU per ampoule, based on their respective calibration by in vivo bioassay in terms of ISU. These assignments of unitage maintain continuity of unitage for preparations in therapeutic use and also appear to be consistent with one another.