Some important vaccines, including ones against tuberculosis (Tb) and foot-and-mouth disease (FMD), are not used in many developed countries because there is no simple way to differentiate between vaccine-induced immunity and infection. Eliminating major protein antigens has not succeeded in the cases of Tb, FMD and some other vaccine preparations. Instead of eliminating antigens, novel antigens can be added to vaccines in order to induce a unique immunological response proving that the vaccine has been delivered. Methods for producing such unique antigens are presented and preliminary test results reported. The technique may also be useful for immunologically "branding" animals.