In the early 1960s, working-class preparative ultracentrifuges were becoming available, and we were able to get one. This represented a ''brute-force'' approach to virus concentration, with as much as 100,0009g to sediment the virus. The forces were massive, but there was a tendency for the sediment to stir back into the supernatant as the rotor decelerated, even with the brake off. We determined that a 2% solution of gelatin was solid at 4°C and liquid at 20°C. This was used to install a 0.1-ml ''trap'' at the point in the tube farthest from the axis of rotation. The rotor and sample were pre-chilled, so that the trap remained solid until the end of the run. Then, the cap was removed from the tube and the supernatant poured off; the trap was liquefied and collected in a small volume of sterile diluent. This was quantitatively efficient, but the volumes of virus suspension that could be processed in a 1-2-h run were relatively limited (Cliver and Yeatman 1965; Gibbs and Cliver 1965) . There was also the problem of an occasional ''catastrophic disassembly event''-the ultracentrifuge manufacturer's euphemism for disintegration of the rotor at speed. This happened twice in our ultracentrifuge; although no one was hurt, cleaning, disinfection, and reconstruction were very problematic.