Abstract
It has been found by immune electron microscopy that rotavirus-infected faeces, calf or human, contain an antigenic subunit associated with the inner of the two virus capsids. This internal component represents the group specific antigen for the rotavirus group and the subunit reacts with both homologous and heterologous antiserum. It can therefore be used in diagnostic tests and in this paper its use as a reagent for immunodiffusion is described.