by Mary Sawyer, Masoud Hosseini, Carrie Schore, and Bennie I. Osburn from School of Veterinary Medicine, University of California, Davis, department of veterinary pathology, microbiology and immunology; Joseph Vu, Baxter biosciences division; Krystyna Trzepla-Nabaglo, Celilia Pina, and Manuel Lagunas-Solar from the laser unit, Crocker Nuclear Laboratory, University of California, Davis; and Wayne Smith, department of population health and reproduction at the University of California, Davis New evidence suggests the West Nile virus can be transmitted during blood transfusion from an asymptomatic donor. However, it is now possible to use pulsed ultraviolet laser light to inactivate viruses such as West Nile at large production scales. These experimental data show that using laser light on virus-treated media can render biological products free of contaminating viruses without compromising the biological activity essential to cell cultures.