Suitable reaction conditions and oligonucleotide primers were sought for the detection of Brucella melitensis and Brucella abortus by the polymerase chain reaction. Primers were chosen from within the coding sequence of a gene encoding a 31 kDa B. abortus antigen. The test was shown to be sensitive, and specificity was demonstrated using DNA derived from a panel of Gram-negative pathogens. There was no detectable difference between B. melitensis and B. abortus in the sensitivity of the reaction or in the size of the amplification product. The technique should be applicable in the diagnosis of brucellosis.