The seroprevalence of Bartonella henselae among veterinary professionals in Japan was investigated by means of an immunoperoxidase (IP) test that used protein A-horseradish peroxidase conjugate. Sera were obtained from 233 veterinary professionals in the Tokyo and Chiba areas. As negative control group, sera from 155 healthy individuals (all medical students) were used. As positive control group, sera from 5 patients highly suspected of cat scratch disease (CSD) by clinical symptoms were tested. Serum antibody titers of > or = 200 to B. henselae were presumed seropositive, because the titer at which about 95.5% of all healthy individuals (148 of the 155) were negative, and 2 of the 5 suspected CSD patients' serum antibody titers were > or = 200. Of the individuals in the veterinary professionals group tested, 35 of the 233 (15.0%) were seropositive for B. henselae. Females were nearly twice as likely as males to have antibodies to B. henselae in the veterinary professionals group. Our data suggest that Japanese veterinary professionals, and in particular younger females who are veterinary assistants and animal beauticians are more often infected by B. henselae.