Foot-and-mouth disease virus (FMDV) was inactivated by ethylenimine (EI) at three concentrations and two temperatures. Comparison of inactivation kinetics and the antigenic and immunogenic potency of EI and N -acetylethylenimine (AEI)-inactivated FMDV indicates that EI has nearly optimal characteristics as an inactivant for FMDV vaccine preparation. Although AEI-inactivated FMDV has proved to be a potent specific immunogen, an equivalent percentage of EI inactivated FMDV at substantially faster rates and produced an equally potent immunogen. In addition, EI inactivated FMDV at rates that were essentially linear throughout the loss of nearly all measurable infectivity.