Transfer of resistance genes from bacteria from food producing animals to human pathogens is a potential risk to human health. The aim of this study was to determine in vivo transfer of a plasmid harbouring a class 1 integron containing gene cassettes dfrA1-aadA1 from Salmonella to Escherichia coli and the influence of the use of antimicrobials on this transfer. Thirty four-day-old SPF chickens colonized with E. coli K12 were divided into 3 groups of 10 animals each, and placed in separate isolators. Eleven days after inoculation with E. coli K12 the chickens were inoculated orally with 10(4)CFU of S. enterica spp. enterica serovar Typhimurium containing a plasmid harbouring a class 1 integron with gene cassettes dfrA1-aadA1. Two days after the administration of S. Typhimurium 1 group was treated orally with doxycycline, 1 group orally with trimethoprim/sulphamethoxazole and 1 group remained untreated (control group). E. coli K12, S. Typhimurium and the transconjugants were isolated from cloacal samples on selective MacConkey agar plates. Transfer of the plasmid was confirmed by plasmid characterization, PCR, PFGE and hybridization. Plasmid mediated transfer of a class 1 integron was observed almost immediately after inoculation with S. Typhimurium. Treatment of the chickens with antibiotics had neither a positive nor a negative effect on the transfer rates. In addition to the resistance genes located on the integron, resistance genes encoding for tetracycline and amoxicillin resistance transferred from the donor strain as well. The resistance genes and the integron were located on a 130 kb sized IncFIB plasmid. Our data demonstrate in vivo transfer of an IncFIB plasmid harbouring a class 1 integron containing gene cassettes dfrA1-aadA1 from Salmonella to E. coli, with or without selective pressure of antibiotics in chickens.