Although the caliciviruses - San Miguel sealion virus, vesicular exanthema virus of swine and feline calicivirus - have been considered a provisional genus of the picornaviridae family (Melnick et al, 1974), as more information becomes available, especially in the area of molecular biology, it becomes increasingly clear that these are not true picornaviruses and hence the suggestion by various workers that these viruses be regarded as a separate virus family. The aim of this study, therefore, was to extend our knowledge of feline caliciviruses: in particular, to examine the structure of the calicivirus particle and the way in which it is assembled. There were four main intentions: first, to determine whether the viral capsid was constructed from smaller subunits; secondly, to identify such subunit structures in FCV-infected cells and examine the roles they play in virion assembly; thirdly, to study and compare the antigenic properties of the whole virus particle relative to a major virus subunit; and fourthly, if both particles showed identical antigenic determinants, to exploit the potential of the subunit component as a source of material for vaccine production.