In Pseudomonas mallei, spontaneous mutants (Tra- mutants) of the plasmids RP4 and R68.45, losing the ability to transfer at conjugation are formed. The plasmids RP4 and R68.45 with Tra(+)-phenotype caused a decrease in P. mallei virulence for laboratory animals. At the same time, Tra- mutants of these plasmids do not affect P. mallei virulence. The insertion of DNA fragment of about 1900 bp into the plasmid transfer gene regions (tra-2-tra-3) gave rise to RP4 and R68.45 tra mutations detectable on examination.