Two preparations of human growth hormone (hGH) were prepared as candidates for the International Standard for Human Growth Hormone for Bioassay and were studied by 22 laboratories in 10 countries in an international collaborative study. The 2 candidate preparations, freeze-dried in ampoules coded 80/505 and 80/521, were assayed against the International Standard for Growth Hormone, bovine, for Bioassay (ISbGH), by in vivo assays; against the International Reference Preparation of Growth Hormone, human, for Immunoassay (IRP hGH), by receptor-, immunoassays and other in vitro methods; and against each other by various methods. Both preparations contained the 2 recognized main growth hormone components (22 kDa and 20 kDa forms) and other components, but that in ampoules coded 80/505 had less deamidated hGH and contaminant hormones and pyrogen than that in ampoules coded 80/521. The estimates by in vivo bioassays, using immature hypophysectomized rats, of the potency of 80/505 in terms of the IS bGH were heterogeneous (1-6 IU/ampoule), probably because of the dissimilarity of the preparations of bovine and human GH. Estimates with receptor- and immunoassays of 80/505 against the IRP hGH were also heterogeneous (3-9 IU/ampoule). Nevertheless, the majority of estimates from all assays tended to be between 3 and 6 IU/ampoule. Although 80/521 and 80/505 differed in relatively minor respects, assays of one directly against the other gave relative potency estimates with in vivo assays which were significantly different from estimates with in vitro methods. With the agreement of the participants in the study, the WHO Expert Committee on Biological Standardization established the preparation in ampoules coded 80/505 as the First International Standard for Human Growth Hormone for Bioassay, with the defined potency of 4.4 IU/ampoule. This corresponds to an approximate 2.5 IU/mg of hGH extract and maintains reasonable continuity of the unit.