A batch of partially purified equine chorionic gonadotrophin (eCG NZY-01) was freeze-dried in vials of 500 iu and characterized in a large number of in vivo and in vitro assays. These assays were performed in eight laboratories in seven countries (Argentina, Belgium, England, France, Germany, Holland and USA). Four of these laboratories were in universities or government research organizations and four were those of pharmaceutical companies. Good correlation between in vivo and in vitro assays was found for unknown samples and for commercial preparations from different sources when performed against eCG NZY-01 as the standard. This result suggests that, in contrast to the WHO IRP2 international standard, the eCG NZY-01 preparation contains all eCG isoforms in proportions roughly similar to those found in serum and commercial preparations. The consistency of the data when using eCG NZY-01 as the reference in all types of in vivo and in vitro assays validates the wide use of in vitro assays which are cheaper, quicker and ethically preferable to in vivo assays. Vials of 500 iu eCG NZY-01 are available to researchers and manufacturers.