Measles and canine distemper are caused by serologically related viruses. Although dogs immunized with measles virus (MV) do not elicit canine distemper virus (CDV) neutralizing antibodies, they are protected against the fatal disease. To investigate the potential role of the MV antigens in protection against CDV, we have immunized mice with vaccinia virus (VV) recombinants expressing the MV haemagglutinin (HA), fusion (F), nucleoprotein (NP) and matrix (M) antigens and challenged them with CDV. A partial protection was observed with the VV recombinants expressing the F, NP and M antigens, but not the HA. In contrast, immunization with a VV recombinant expressing the CDV F protein completely protected mice from CDV.