|Foot-and-mouth disease (FMD) is a highly contagious viral disease of cloven-hoofed animals. After the outbreak of serotype O1 FMD virus in Taiwan in 1997, pigs were extensively vaccinated. To investigate the development of antibody isotypes in response to FMD vaccine and the influence of maternally derived antibodies on the production of antibody isotypes in vaccinated piglets, sera were collected over a five month period from twenty vaccinated sows (from two farms) after they were boosted with the FMD vaccine. Sera were also collected from 18 one-month-old piglets from each before and after they were vaccinated. Piglets at farm A were vaccinated when they were three months old; those at farm B were vaccinated at both two and three months. As a control, serum samples were collected from 30 FMD virus-free pigs and 30 FMD vaccinated pigs without booster injection at two additional farms. We analyzed the FMD virus proteins, which contain the 66kD and 13kD antigens recognized by serum antibodies of vaccinated pigs, to find that the IgG1 antibody level increased significantly in vaccinated sows receiving booster injections. The IgG2 antibody was enhanced only in sows at farm A. The IgA levels at both farms were not significantly higher compared to sows without booster injections, but the IgM levels at farm A were constant, which was significantly higher than at farm B or among pigs with no booster. Pigs without booster injections had significantly higher IgG1, IgG2 and IgA levels than those of FMD virus-free pigs, but the IgM antibody levels did not differ. The maternally derived antibodies were IgG1 and IgG2. Serum IgG1 and IgG2 antibodies increased significantly only if piglets were immunized twice. However, the levels of IgM and IgA in piglets did not vary with frequency of vaccination.