A Brucella melitensis infection involved three persons. The infections were acquired by contact with a culture suspension that had been spilled during a laboratory exercise. One of the patients had only indirect contact with the spilled culture, probably contact with contaminated fomites. Another of the patients developed a clinical remission before treatment with antibiotics and relapsed after six months. This patient also had a positive bone marrow culture when blood cultures were negative. The type of antibody response persisting in the sera of convalescents was studied by treating the sera with mercaptoethanol and rabbit antihuman IgM sera. In the patient who relapsed, IgG antibody predominated, whereas IgM persisted for over two years in the patient without relapsing disease.