The European Pharmacopoeia (Ph. Eur.) monograph for varicella vaccine (live) (0648) requires a vial of an appropriate reference material to be titred in triplicate to validate each assay and the virus concentration of the reference preparation is monitored using a control chart to determine the assay consistency. An international collaborative study involving 9 participants from 7 countries and including both OMCLs and manufacturers was carried out to establish a common reference material for this purpose and establish a Ph. Eur. Biological Reference Preparation. Two candidate reference preparations (X and Y), obtained from 2 different EU manufacturers, were assayed by the participants using their in-house PFU assay methods. Both candidates were found to be suitable for this purpose. Based on logistical considerations, candidate X (4.37 log(10)0 PFU/vial) has been established as BRP batch 1 of varicella vaccine (live) and was adopted at the June 2009 session of the European Pharmacopoeia Commission for immediate use. Candidate Y (3.82 log(10) PFU/vial) will be established as BRP batch 2 upon depletion of BRP batch 1 provided that the stability data supports this.