THE members of the myxovirus group may be divided into two classes on the basis of the form of their particles, and this division coincides with that made on biological criteria1,2. The two groups are the influenza group, on one hand, and the para-influenza group, on the other. The first group includes influenza A, B and C, fowl plague and virus N; the virion is small, not easily disrupted, bears conspicuous spikes, and contains an internal helical ribonucleoprotein (nucleoprotein antigen, or NP-antigen). This, on disruption of the particle, is seen to be 90-100 Å in diameter, to occur in lengths generally less than 1000 Å, and to lack the conspicuous herringbone appearance often seen in the corresponding structure of the other group. This structure has not yet been seen in the viria of influenza C (ref. 3).