In many parts of the world contact with infected livestock may involve a serious risk of the spread of human brucellosis. Partial control of bovine brucellosis has been achieved by slaughter of infected herds and immunization of cattle with Brucella abortus strain 19 living vaccine. However, in areas where such measures are unpractical there remains a need for protection of humans. This study compares the safety of two living Brucella vaccine preparations in human volunteers.Some 32 healthy male volunteers with no evidence of past exposure to brucellosis were divided into two comparable groups; one group received 19-BA vaccine derived from Br. abortus and the other received Rev 1 vaccine derived from Br. melitensis. Detailed studies over a six-month period of the clinical effects, bacteraemia, Brucella agglutinin response and dermal hypersensitivity revealed striking differences between the two groups. Two of the 16 men in the 19-BA group developed acute brucellosis, and another had a positive blood culture. In the Rev 1 group, 11 of 16 developed acute brucellosis, and brucellae were recovered from 12.All 32 men developed Brucella agglutinins, the Rev 1 group having higher titres. Dermal hypersensitivity occurred in all of the Rev. 1 group but in only nine of the 19-BA group.Tetracycline treatment in all the Rev 1 group and in the two brucellosis cases in the 19-BA group resulted in complete recovery.The authors conclude from this study that neither the Rev 1 vaccine nor the 19-BA vaccine inoculated subcutaneously is sufficiently safe in the dosage used to warrant being used for vaccination of humans for prophylactic purposes.