Much subclinical infection with Brucella abortus affects the dairy farming community, slaughterhouse workers, and veterinary surgeons. High titres of antibody to B. abortus by the saline agglutination, antihuman globulin, or complement fixation test may be shown in subjects from the first two groups and generally there are no symptoms or signs of brucella infection whatever. Veterinary surgeons are prone to subclinical infection and young practitioners frequently display high titres against the usual tests at the same time apparently enjoying good health. Repeated reinfection of this group, however, may result in hypersensitivity to B. abortus and bouts of ill health at intervals may be due to this. Infection seems to come more often from contact with infected material than by drinking untreated milk, particularly in the herdsman, slaughterhouse worker, and veterinary surgeon. A doctor investigating illness of a patient from the dairy farming community and allied occupations should not accept a high titre against B. abortus as the only clue to diagnosis of the patient's condition.