The buoyant density of MTV in several different gradient solutions was determined, using a single pool of MTV-infected milk as the source of virus. Increased ultraviolet absorbance and detectable MTV B-particle antigenicity correlated with the positions of discrete light-scattering bands formed during centrifugation. MTV isodensities were found to be similar to those reported for other RNA tumor viruses. Centrifugation of MTV-positive preparations in preformed gradients containing sucrose or potassium tartrate resulted in the formation of two distinct light-scattering bands. Electron microscopic examination showed that only those bands at the higher buoyant density contained characteristic MTV particles. The bands of lower isodensity consisted of viruslike particles having considerable variability in size and shape, which may represent incomplete MTV particles. The properties of isopycnically centrifuged MTV-free milk samples were clearly distinguishable from those of MTV-positive samples; no discrete light-scattering bands were formed, and no viruslike particles were observed in electron micrographs.
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