Five preparations of interleukin-3 (IL-3) have been evaluated by 28 laboratories in 12 countries for their suitability to serve as an international standard for this material in a joint international collaborative study for IL-3 and interleukin-4 (IL-4). The preparations were assayed in a wide range of in vitro bioassays and immunoassays. It is clear from the biological assays contributed to this study that different recombinant preparations of IL-3 can have very different biological specific activities, including those from the same source (i.e., E. coli). Biological assays of IL-3 were significantly more consistent in their estimates of levels of IL-3 than the immunoassays, suggesting an unusual pattern of epitope recognition amongst the antibodies included in the immunoassays. This study also illustrates the point that the level of cytokine measured by immunoassay does not necessarily reflect the biological potency of the cytokine. On the basis of results reported here, with the agreement of the participants of the study and with the authorisation of the Expert Committee on Biological Standardization (ECBS) of the World Health Organization (WHO) the preparation of IL-3 ( 91 510 ) was established as the international standard for interleukin-3 with an assigned unitage of 1700 IU/ampoule.