SUMMARY The proteins of the virus particles, empty capsids, and I2S subunits of foot-and- mouth disease virus were compared by polyacrylamide gel electrophoresis in neutral phosphate buffer with SDS and in I M-propionic acid with urea. In the first system the tool. wt. of the observed components were estimated and in the second system proteins with similar mol. wt. were resolved by their different charges. The proteins were either denatured by urea, reduced by mercaptoethanol, or oxidized by performic acid. They were dissociated completely only by reduction or oxidation. By denaturation with 8 M-urea, dimers with rnol. wt. of about 58ooo were observed. In propionic acid-urea electrophoresis, each of the three virus types A2 Spain, Ol Kaufbeuren and C Upper Bavaria showed three main proteins (VP I, VP 2, VP 3) and a fourth fast-migrating protein, VP4. The rates of migration of VP I and VP 3 were approx, the same in all types, whereas VP 2 showed differences between types. The component VP 4 was not present in the empty capsids or in the I2 S subunits. On elution of the proteins from the gets used in propionic acid-urea electrophoresis followed by re-electrophoresis in both systems, it was found that the relative electrophoretic migration rates of VP I and VP 3 were opposed in the two systems. The oxidized proteins VP I, VP 2, and VP 3 had similar mol. wt. of about 26 ooo to 28 ooo but varied in charge. Of the native virus, only one protein was split by trypsin, leaving two detectable frag- ments of 95oo and 185o0 daltons.