A thin sectioning procedure was used to detect and enumerate oncornavirus particles in tumour-cell culture fluids, tumour homogenates, mouse blood plasma, mouse and human milk specimens, and density gradient fractions of these specimens. Oncornaviruses studied included mouse mammary tumour, murine sarcoma, ESP-1, and RD-114 viruses. In this procedure particles were sedimented on to small membrane filter discs in the ultracentrifuge using inexpensive commercially available adapters and tubes. Particles in cross sections of the disc were counted and their total number determined by relating the effective surface area of a field to the surface area of the entire membrane disc. Particles may be reliably identified and counted in preparations containing predominately cellular debris. Linear dose response plots were obtained in serial dilution experiments utilising vaccinia virus, adenovirus type 2, and murine sarcoma virus, demonstrating the reliability of the procedure and its wide applicability. The counts obtained for adenovirus and vaccinia virus preparations were comparable to counts obtained for the same preparations in other laboratories by established methods. Statistically reliable counts have been obtained using sample vol. of 0·01 ml or less.


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