Another important development at that time was the introduction of cellulose-ester membrane filters, which could be stored in the dry state and sterilized by Food Environ Virol (2010) 2:1-23 5 autoclaving. Cellulose nitrate (gun cotton) had long been in use as a component of smokeless gunpowder-no accidents seem to have occurred with the filters. Some of the filters were said to comprise pure cellulose nitrate, while others were composed of mixed esters of cellulose, with nitrates predominating. These represented a huge advantage from the standpoints of convenience and reliability. They were produced in a great array of nominal pore sizes, based on the size particles that would pass through and on the pressure required to force air through water-wet filter. The pores themselves were not uniform, and the filters resembled a sponge 150 lm thick, but their ability to retain or reject particles larger than their nominal pore size was phenomenal. A distinguished diagnostic virologist, G. D. Hsiung, soon put this rejection capability to work in classifying unknown viruses (Hsiung 1965; Hsiung and Henderson 1964) . According to her scheme, small viruses would pass a nominal 50-nm filter, medium viruses would pass a nominal 100-nm filter but not the 50 nm filter, and large viruses were completely retained by a filter of 100-nm porosity. She noted that these properties were qualitative, in that small viruses, for example, would be found in the filtrate of the 50-nm filter, but a large proportion of the viral particles were retained in the filter matrix. We were able to show that adding a small proportion of animal serum to the virus suspension obviated such retention (Cliver 1965a) .