Cross-reactions between two strains of foot-and-mouth disease virus (FMDV) belonging to different serotypes (A and O) were studied with intact virus and virus subunits and antisera produced against the isolated structural proteins. Anti-VP1 type O serum showed cross-reactive neutralizing activity, in contrast to the sera raised against intact virus type O, whereas anti-VP1 type A serum only neutralized homologous virus. Anti-VP2, VP3 and VP4 did not show neutralizing activity. In the enzyme-linked immunosorbent assay and radio-immunoassay anti-VP1, -VP2, and -VP3 sera reacted with the 12S virus subunits of both serotypes. No activity was obtained against VP4. Competition experiments with virus subunits of virus type A and O show that anti-VP1 serum is the most type-specific. Anti-VP2 serum is completely cross-reactive, while anti-VP3 serum reacted in an intermediate way. The identical reactions obtained with anti-VP2 sera and the homologous and heterologous virus subunits suggest that the exposed VP2 antigens are identical.