Methods are disclosed for sterilizing tissue to reduce the level of one or more active biological contaminants or pathogens therein, such as viruses, bacteria, (including inter- and intracellular bacteria, such as mycoplasmas, ureaplasmas, nanobacteria, chlamydia, rickettsias), yeasts, molds, fungi, prions or similar agents responsible, alone or in combination, for TSEs and/or single or multicellular parasites. The methods involve sterilizing one or more tissues with irradiation.