A technique is described for embedding tissue culture cells that have been adsorbed or grown on Millipore filters. The acetone used during the embedding process rendered the filters transparent so that specific areas or cells could be chosen with the aid of the light microscope. Lymphoblastoid cells processed on the filters possessed well-defined plasma membranes and microvilli which were rarely present in cells from parallel cultures that were prepared by pelleting in the centrifuge. Fibroblast cells grown on filters retained their elongated appearance, in contrast to the rounded cells in pelleted preparations. Millipore filters were also used as a means of embedding virus pellets for sectioning. Preparations containing as few as 4 x 108 virus particles were suitable for study by the filter technique. Crude tissue-culture harvests of vaccinia virus and purified preparations of Rauscher murine leukemia and adeno-satellite viruses were successfully examined.

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