Two novel Brucella abortus proteins were isolated from B. abortus strain RB51 and their immunological properties were determined. These proteins precipitated in the 40-60% saturated concentration range of ammonium sulphate and had a molecular mass of 32.2 kDa and 22.9 kDa, respectively. Both were able to induce a strong in-vitro blast transformation in lymphoid cells obtained from mice previously sensitised with a crude brucella protein extract. The protection studies showed that the 22.9-kDa protein used as a protective immunogen was as effective as the live B. abortus RB51 vaccine but the 32.2-kDa protein had a poor protective effect under similar conditions. The amino-terminal sequence of the 22.9-kDa and 32.2-kDa proteins was determined and analysed in a database. The lack of homology with other known B. abortus proteins indicated that both proteins were novel antigens.