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The reactions of foot-and-mouth disease virus with IgG and IgM have been studied by agar precipitin tests and by electron microscopy. The 140s component produces a precipitin band with each antibody class. It also produces a band with IgG which has been absorbed with 12s virus protein sub-units or trypsin-treated virus, providing evidence for the presence of more than one type of combining site. Trypsin-treated virus also gives a band with IgG and with IgG absorbed with 12s protein sub-units, indicating the presence of a third site. Electron microscopy shows that IgG reacts with the entire surface of the virus particle, producing complexes in which the outline of the virus particles is obscure. In contrast, IgM or IgG which has been absorbed with 12s protein sub-units or trypsin-treated virus forms complexes in which attachment is at regularly spaced sites on the virus surface. Trypsin-treated virus also produces complexes of this type with IgG absorbed with 12s protein sub-units. The virus thus appears to possess at least three types of combining site, one on the faces of the particle and the others at regularly spaced intervals, probably at the vertices.