A collaborative study was run by the European Directorate for the Quality of Medicines and HealthCare (EDQM) under the aegis of the Biological Standardisation Programme (BSP) to establish replacement batches of the current Prekallikrein activator in albumin Biological Reference Preparation (BRP) batch 1, the stocks of which were dwindling. Candidate BRP replacement batch 2 and batch 3 were assayed against the 2nd World Health Organization International Standard for Prekallikrein activator, human (2nd IS) and the Prekallikrein activator in albumin BRP batch 1.The candidate batches were manufactured from the same starting material as the current Biological Reference Preparation and the 2nd IS. They consisted of a 20 % solution of albumin lyophilised under the same conditions as the Prekallikrein activator in albumin BRP batch 1. Sixteen laboratories participated in the collaborative study and were requested to assay the candidates by their routine method, complying with the European Pharmacopoeia (Ph. Eur.) general method 2.6.15 for the determination of prekallikrein activator content. A central statistical analysis was performed at the EDQM using in-house calculations of prekallikrein activator contents provided by the participating laboratories. On the basis of the results of this study, which confirmed the assigned potency of 29 IU/vial of Prekallikrein activator in albumin BRP batch 1, the 2 candidate materials were assigned a potency of 30 IU/vial. The 2 candidates were adopted by the Ph. Eur. Commission in March 2008 as Ph. Eur. Prekallikrein activator in albumin Biological Reference Preparation batch 2 and batch 3.