SUMMARYThe important role of plasmid genes in assessing virulence for BALB/c mice in salmonella, and the difficulty of using standard techniques to detect them, led us to develop a detection method by gene amplification.One hundred and forty-three strains (71 serovars) of salmonella and 35 strains of other species were tested using specific oligonucleotide primers. The amplification products were identified by a specific oligonucleotide probe. Forty-nine salmonella strains from ten serovars (S. abortus ovis, S. choleraesuis, S. dublin, S. enteritidis, S. gallinarum/pullorum, S. hessarek, S. typhimurium, S. IIIa 48: Z4, Z23, S. IV43:z4, Z23: -, S. V 28: a: -) produced a positive and specific response.Because of various origins of the strains possessing the gene sought and the diversity of the responses, both from one serovar to another and in the same serovar, this search has its place among the epidemiological markers in general use. This method appears well suited to the research and detection of plasmid genes associated with mouse virulence in salmonella.