We also tested a procedure whereby the virus suspension was sealed in dialysis tubing and immersed in a 50% solution of polyethylene glycol (PEG). This could be done with virtually any virus and any number of samples (PEG was inexpensive), but harvesting the samples typically awaited the following day (Cliver 1967a; Gibbs and Cliver 1965) . There was the additional problem that if PEG contaminated the concentrated sample, the sample would intoxicate the inoculated cell culture. It was necessary to rinse the outside of the dialysis tubing carefully before opening it; we also devised a wringer apparatus that would force the concentrated sample to the open top of the dialysis tube for aseptic collection.