The World Health Organization (WHO) Expert Committee on Biological Standardization (ECBS) has recognized the need for an International Standard for Insulin-like Growth Factor-1 (IGF-1) for the calibration of immunoassays and for the monitoring of the content of therapeutic products. The objective of the study reported here was the characterization of a candidate standard for IGF-1 in an international collaborative study carried out by 18 laboratories in nine countries, by comparison with (i) a primary calibrant characterized by amino acid analysis and UV spectroscopy, and (ii) the existing International Reference Reagent coded 87/518 by HPLC, immunoassay and bioassay. The study was designed as follows: Phase I involved the establishment of a primary calibrant of rhIGF-1, containing approximately 1.0mg rhIGF-1 per vial. A defined value was assigned to the primary calibrant by amino acid analysis (AAA) and UV spectroscopy. Phase II involved calibration of the candidate standard in terms of the primary calibrant by HPLC, with confirmatory data from immunoassay and bioassay. Results from Phase I confirmed the primary calibrant as containing 1.045 mg per vial. Although there was some variability among laboratory estimates of IGF-1 in the proposed standard using the different methods in Phase II, the estimates by the various methods were in broad agreement. On the basis of the results reported here, the World Health Organization (WHO) has established the preparation coded 02/254 as the First International Standard for Insulin-like Growth Factor-1, human, recombinant, for immunoassay with an assigned content of 8.50 microg per ampoule. Details of how to order the standard can be found at www.nibsc.ac.uk.