Transfer of conjugative transposon Tn1545 from Enterococcus faecalis to Listeria monocytogenes was studied in vitro and in vivo. Tn1545 transferred following filter mating at a frequency of 2.5 x 10(-7) transconjugants per donor colony. A 20-fold increase in transfer frequency was observed when matings were performed in the presence of a subinhibitory concentration of tetracycline. The frequency of in vivo transfer of Tn1545, expressed as the number of transconjugants per donor cell extracted from the intestines of the gnotobiotic mice after 35 days of experiment, was 1.1 x 10(-8). Presence of a low concentration of tetracycline in the drinking water increased this frequency 10-fold.