Abstract: A novel irradiation process and products made thereby. The process treats biological media such as blood fractions, genetically engineered protein products and vaccine preparations. The process photolyzes nucleic acids in preference to proteins in the media, e.g., it inactivates DNA- or RNA-containing pathogens while leaving the proteins substantially intact or functional. In general, the process comprises irradiating the medium with pulsed light of wavelength and flux selected so that (1) the nucleic acids in their ground state absorb radiation and thereby rise to an excited state or states, (2) the nucleic acids in their excited states absorb radiation and thereby rise to higher energy states and undergo photolysis, and (3) the proteins in their ground or their excited states do not absorb sufficient radiation to undergo substantial photolysis.