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This communication describes a method whereby avian rnyeloblastosis virus ( AMV) was rapidly purified from high-titer viremic plasma [over 10l2 particles per ml by enzyme assay ( l ) ] by using rate-zonal centrifugation in linear density gradients of Ficoll (2, 3). The use of Ficoll was suggested by studies that employed Ficoll to isolate murine leukemia virus (4), and mammary tumor virus (5, 6), a type B RNA t u m r virus. Our procedure for AMV was devised in order to (a) minimize the time of purification, (b) avoid the objection that cellular vesicles and AMV band together in equilibrium centrifugation, (c) assess the association of AMV and 18s and 28s species of RNA (7-9) with a different technique of AMV purification, and (d) avoid exposure of intact AMV to high osmotic pressure from glycerol, salt, or sucrose density gradients which hinder production of the AMV o r e component during surfactant treatment (10).AMV isolated by this method had a threefold increase in specific activity of adenosine triphosphatase [ATPase, an enzyme of the viral envelope ( 1 1) ] during purification, and a high degree of morphological purity by electron microscopy. A linear relationship was found to exist between the turbidity of a, purified AMV sample and its protein content and ATPase activity. Thus, an estimate of the concentration of AMV could rapidly be made by simply determining the percentage transmission of a virus preparation. Ribosmal-like RNA was present in AMV purified by this method.Methods. Viremic blood was obtained from chicks as previousIy described (10).

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