The creation of bacterial mutants by transposon mutagenesis has facilitated the identification of regulatory and structural genes. In the case of B. abortus the number of reported transposon mutants created by mating or P1 infection has been relatively small. We studied the conditions necessary to introduce Tn5 bearing a kanamycin resistance gene (KnR) into B. abortus by electroporation. The highest frequency of Tn5 transposition was obtained using B. abortus 2308 harvested at a density of 5.2 x 10(8) cells/ml; 0.5 microgram of plasmid was electroporated for 10 ms at 625 V (equivalent to 12.5 kV/cm). The frequency of Tn5 transposition obtained under optimum conditions was estimated to be around 18-20 insertions per 10(10) Brucella. The phase of growth (or the number of generations) had a strong influence on the frequency of transposition. Dot blot analysis confirmed that all KnR clones appearing after 4 days of incubation at 37 degrees C carried Tn5 in their genomes. Furthermore, the randomnes of Tn5 insertion was verified by Southern analysis of chromosomal DNA extracted from knR clones and hybridized with labeled Tn5.