This study compares the immune responses and protection induced by intra-typic heterologous vaccination with that induced by homologous vaccination against challenge with foot-and-mouth disease virus (FMDV). Humoral and cell-mediated immune responses and protection against challenge with FMDV O Taiwan were examined in a non-vaccinated group, a group vaccinated with O Taiwan FMD vaccine and a group vaccinated with O Manisa FMD vaccine. Five pigs from each group were challenged with FMDV type O Taiwan 14 days after vaccination and five other pigs were contact-exposed to the inoculated pigs. Both homologous and heterologous vaccination protected against challenge with FMDV O Taiwan at 2 weeks after vaccination. In the heterologous vaccinated group, cross-neutralizing antibody titres against O Taiwan could be detected although the ratio 'r(1)' was 0.4, which was significantly smaller than the critical r-value. Cell-mediated immune responses were detected after both homologous and heterologous vaccination. Virus-induced in vitro lymphocyte (cross-) proliferation and production of both a Th1-type (IFN-gamma) and a Th2-type (IL-10) cytokine response were demonstrated in cultures of peripheral blood mononuclear cells (PBMC). The findings show that heterologous (emergency) vaccination can prevent clinical disease and shedding of virus. The induction of the cell-mediated immune responses after (heterologous) vaccination needs more research but data on these responses might provide additional tools for both vaccine choice and vaccine development.