Two forms of a baculovirus, Trichoplusia ni nuclear polyhedrosis virus, one released at the plasma membrane of cells (cell released virus: CRV) and the other derived from polyhedra (polyhedron derived virus: PDV) occur naturally. Antisera raised against the two forms specifically neutralised the homologous form. The envelope proteins of the two forms were shown to be the major proteins involved in serum neutralisation and to be responsible for the antigenic diversity of the virus forms.