To evaluate the immune response induced by Japanese rabies vaccine for veterinary use as international units (IU), we measured levels of rabies antibody in serum samples from dogs by the rapid fluorescent focus inhibition test (RFFIT). In dogs immunized with a reference vaccine (potency level of 3.1 IU/ml), prepared by the same method as that used to produce commercial vaccine, and its dilutions (1 : 2 or 1 : 4), neutralizing-antibody levels increased to 1.0-2.0 IU/ml over a period of 1 month and then decreased to 0.2-1.5 IU/ml over a period of 1 year after the first vaccination and showed a remarkable increase to 12-47 IU/ml after the second vaccination. Sixty-five (74.7%) of the 87 serum samples from domestic dogs that were tested were seropositive (> or =0.1 IU/ml). However, the seropositive rate in dogs less than 1-year old at the time of vaccination was low (57.1%), and the antibody levels in these dogs were not sufficiently high for the rabies antibody titre in serum to be maintained for 1-year. Levels of rabies antibody in all serum samples were also measured by the virus neutralizing test (VNT), and a strong correlation (r > 0.95) was found between the results of the RFFIT and those of the VNT.