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Three strains of foot-and-mouth disease virus were shown to contain significant amounts of naturally occurring 75 S, empty particles as well as the infectious, 140 S full particles. One of these strains — A Pando (1970) — was studied in detail.

The empty particles from this virus strain were shown to have an observed sedimentation coefficient of 67S in 0.04m phosphate buffer; they were labile in SDS, non-infectious and probably RNA-free and, on heating, they broke down to 12 S subunits as did the 140 S particles. The empty particles differed from the full particles in their polypeptide composition since they contained VP₀, but there was no evidence for a diminished content of VP₄.

The 75 S particles were shown to be present in significant amounts and to be stable to AEI inactivation. At 4° C they were stable for at least two years. In guinea pigs they were as immunogenic as the 140 S particles …