Following the discovery of Cryptosporidium oocysts in some of the raw water sources supplying Clay Lane Water Treatment Works, it was decided to build an ultrafiltration plant (at 160 Ml/d, the largest in the world) to mitigate the risk of this passing into the treated water. This decision was confirmed by the designation of Clay Lane as a site "at significant risk" from Cryptosporidium following the application of the Cryptosporidium regulations in 1999. Since the plant was commissioned in April 2001, it has enabled excellent quality water to pass into supply, and this has been backed up by an integrity testing system to ensure that fiber breakages can be detected and dealt with promptly. The ultrafiltration unit has allowed the plant to run at a much higher throughput under high turbidity raw water conditions, without compromising treated water quality. The membrane fouling that does take place appears to be organic in nature and further work is underway to gain a greater understanding of the issue. Includes 4 references, figures.