Candidate foot-and-mouth disease (FMD) DNA vaccines designed to produce viral capsids lacking infectious viral nucleic acid were evaluated. Plasmid DNAs containing a portion of the FMDV genome coding for the capsid precursor protein (P1-2A) and wild-type or mutant viral proteinase 3C (plasmids P12X3C or P12X3C-mut, respectively) were constructed. Cell-free translation reactions programmed with pP12X3C (wild-type 3C) and pP12X3C-mut produced a capsid precursor, but only the reactions programmed with the plasmid encoding the functional proteinase resulted in P1-2A processing and capsid formation. Baby hamster kidney (BHK) cells also produced viral capsid proteins when transfected with these plasmids. Plasmid P12X3C was administered to mice by intramuscular, intradermal, and epithelial (gene gun) inoculations. Anti-FMD virus (FMDV) antibodies were detected by radioimmunoprecipitation (RIP) and plaque reduction neutralization assays only in sera of mice inoculated by using a gene gun. When pP12X3C and pP12X3C-mut were inoculated into mice by using a gene gun, both plasmids elicited an antibody response detectable by RIP but only pP12X3C elicited a neutralizing antibody response. These results suggest that capsid formation in situ is required for effective immunization. Expression and stimulation of an immune response was enhanced by addition of an intron sequence upstream of the coding region, while addition of the FMDV internal ribosome entry site or leader proteinase (L) coding region either had no effect or reduced the immune response.