Following the heparin adulteration crisis, the European Pharmacopoeia (Ph. Eur.) Group of Experts on Biologicals (Group 6) considered a revision of the general chapter 2.7.5. Assay of heparin with regard to the assay of the anticoagulant activity of heparin in order to replace the clotting method with more specific chromogenic methods for anti-IIa and anti-Xa activities. An international collaborative study was carried out in 3 phases under the aegis of the Biological Standardisation Programme (BSP) of the Council of Europe and the European Commission in order to recalibrate Heparin sodium Biological Reference Preparation (BRP) batch 3 for these new assays. Phase 1 confirmed the feasibility of the project, but also indicated that the composition of the buffers affects the assay results, thereby highlighting the importance of using common assay procedures. Phase 2 consisted of a collaborative study involving 15 laboratories to calibrate the anti-IIa and anti-Xa activities of Heparin sodium BRP batch 3. The collaborative study confirmed that Heparin sodium BRP batch 3 is suitable for use as a reference preparation in the proposed chromogenic assays for unfractionated heparin. It also showed that the currently defined acceptance limits (90 % to 111 %) can be maintained in the revised Ph. Eur. texts. Phase 3 of the study collected data on the impact of the new unitage on the release of products marketed in Europe. The data from 5 manufacturers, who each reported results from both the clotting and chromogenic assays for a total of 23 batches, indicated that the replacement of the pharmacopoeial method is unlikely to cause batch release issues. Based on the results of this study, the Ph. Eur. Commission assigned Heparin sodium BRP batch 3 with a potency of 1000 IU/vial for both anti-IIa and anti-Xa activities in the chromogenic assays.