Two distinct types of rotavirus particle were isolated in caesium chloride density gradients. The higher density particle banded at 1-38 g/ml and measured 55 +/- 0-4 nm in diameter while the less dense particle banded at 1-36 g/ml, measured 66 +/- 0-4 nm in diameter and appeared to possess an extra outer capsid layer. Some forms intermediate between the two types were seen. Since the two particles had a similar but not identical morphology and polypeptide composition, they were considered to be different forms of the same virus. Infectivity was associated with the larger particle which contained one more polypeptide than the smaller particle.
Bridger et al. (1976) studied this question.