To investigate the antibody response of the Histophilus somni vaccine in Japanese black calves, 60 calves fed at two farms (Farms A and B) were studied.On Farm A, 32 calves were treated, with 16 calves in the treatment group and 16 in the control group, and on Farm B, 28 calves were treated, with 14 calves in the treatment group and 14 in the control group.In the treatment group, H. somni inactivated vaccine was administered at 4 and 8 wk of age.Blood was collected from all tested calves at 1, 4, 8, 12, 16 and 20 wk of age, and the antibody titer against H. somni was measured.The antibody titer after 8 wk of age in the treatment groups of both farms increased significantly as compared with the antibody titer at 1 and 4 wk of age (P<0.05).In addition, the antibody titers of the treatment groups of both farms were significantly higher than those of the control group (P<0.01).These results suggest that the administration of H. somni inactivated vaccine to Japanese Black calves increases the antibody titer to H. somni.