We investigated the dynamics and duration of antibody titer against Mannheimia haemolytica in Japanese Black calves. Twenty unvaccinated calves from two Japanese Black breeding farms in Kagoshima Prefecture, Japan, were studied. The antibody titer against M. haemolytica reached the lowest level at 8 weeks of age after birth. Calves began producing antibody against M. haemolytica by themselves between 8 and 12 weeks of age. The results of this study might help designing a vaccination program against M. haemolytica for Japanese Black calves.