Following the confirmation that Mr Litvinenko had received a large dose of 210Po, the Health Protection Agency began to prepare for public health monitoring of potentially contaminated people. Alpha counting of urine was the only practicable means of monitoring exposure. As the incident developed it became apparent that the number of samples would be likely to exceed the HPA's capacity. Therefore other UK laboratories were contacted and several agreed to assist. To validate the analyses each potential collaborator analysed a cross check sample that was the remainder from 24 hour samples that had already been measured at the HPA. The results were in good agreement. In the first four weeks of the programme, about 600 samples were collected and all were analysed using the resources in the UK. Later, the quality assurance exercise was extended to include laboratories in the UK and abroad that could potentially become involved in the analysis of samples arising from the incident. The factors underlying the success of the programme are set out. These included having a well founded Quality Assurance system in place during normal circumstances and having pre-existing links at the appropriate working level with other laboratories in the UK and abroad.