The viral and transmissible spongiform encephalopathy (TSE) safety of therapeutics of biological origin (biologicals) is greatly influenced by the nature and degree of variability of the source material and by the mode of purification. Plasma-derived and recombinant DNA products currently have good viral safety records, but challenges remain. In general, large enveloped viruses are easier to remove from biologicals than small 'naked' viruses. Monoclonal antibodies and recombinant DNA biopharmaceuticals are derived from relatively homogeneous source materials and purified by multistep schemes that are robust and amenable to scientific analysis and engineering improvement. Viral clearance is more challenging for blood and cell products, as they are complex and labile. Source selection (e.g. country of origin, deferral for CJD risk factors) currently occupies the front line for ensuring that biologicals are free of TSE agents, but robust methods for their clearance from products are under development.