On May 26-27, 1997, nine persons (a farmer, four veterinary clinicians, and four veterinary students) in Manhattan, Kansas, participated in an attempted vaginal delivery, a cesarean delivery, and a necropsy on a stillborn calf that died because of Brucella abortus infection. The infection was confirmed by isolation of B. abortus from placental and fetal lung tissue cultures. The National Animal Disease Center, U.S. Department of Agriculture (USDA), identified the B. abortus isolate from the calf as the RB51 vaccine strain. RB51 is a live, attenuated strain that was licensed conditionally by the Veterinary Services, Animal and Plant Health Inspection Service, USDA, on February 23, 1996, for vaccination of cattle in the United States. Before 1996, vaccine was made by using the S19 strain. This report describes occupational exposure to animals infected with the RB51 strain and emphasizes the need for surveillance of unintentional exposure of humans to RB51 to assess outcomes of such exposures.