In 2002, the City of Henderson, Nevada, completed construction of one of the first ultraviolet (UV) disinfection facilities in the United States for water treatment. The City of Henderson (City) system was specifically designed to inactivate Cryptosporidium oocysts in a surface water supply. The City's schedule for implementation of advanced disinfection required that the system be designed and constructed prior to the development of specific guidance from the United States Environmental Projection Agency (USEPA). The USEPA is currently developing a draft Ultraviolet Disinfection Guidance Manual (UVDGM) with publication scheduled for Summer 2003. The UVDGM will include a detailed protocol for UV reactor validation testing by biodosimetry, and validation testing will represent the critical step in establishing the operating setpoints and limits for UV systems. Validated reactors will be required for all U.S. UV installations, and it appears that the validation protocol will be sufficiently flexible to allow validation off-site at a testing facility or on-site at the treatment facility. For the City's on-site testing, bioassays were conducted in November 2002 using one of the full-scale reactors to validate the performance capability of the UV equipment. In this paper, the issues associated with the full-scale, on-site validation testing completed at Henderson are described. The paper also describes the testing procedure, the experimental results, and the implications of these results. This paper also summarizes the facilities required for on-site validation and compares the advantages and disadvantages of the options for UV reactor validation. Includes 7 references, tables, figures.