Resistance plasmids were transferred in urine from a multi-drug-resistant Serratia marcescens to Escherichia coli. Transfer of resistance to kanamycin, tetracycline, chloramphenicol, streptomycin, ampicillin, and carbenicillin occurred readily after 4 h of incubation at room temperature (25 degrees C). The urinary catheter collection bag is postulated as a potential site for extraintestinal resistance plasmid transfer in the Enterobacteriaceae, especially for pathogens such as Serratia, which do not frequently colonize the intestinal tract.