Determination of the buoyant density of canine distemper virus by radioassay.

Canine distemper virus was labelled with tritiated uridine and, following precipitation with saturated ammonium sulphate solution, was concentrated 66-fold by centrifugation through a discontinuous sucrose gradient. When this preparation was centrifuged to equilibrium in density gradients of potassium tartrate or sucrose, radioactivity was distributed over the density range 1.218 to 1.180 with a pronounced peak at around 1.195. This corresponded closely to the distribution of infectivity and also to that of virus particles revealed by electron microscopy. In density gradients of caesium chloride, a plateau of radioactivity was present over the density range 1.26 to 1.24 with a peak at around 1.240 but most of the infectivity was limited to the range of 1.24 to 1.22. Since the amount of infectious virus recovered from potassium tartrate was greater than that recovered from the other two materials and the radioactive peak occurred over a narrower density range, it was concluded that potassium tartrate was the material of choice for the isopycnic centrifugation of canine distemper virus.