Most clinical isolates of Yersinia enterocolitica have been reported to be inhibited by a wide variety of antimicrobial agents. This finding indicates that Y. enterocolitica infrequently acquires resistance by gene transfer from other bacteria in nature. In the present study the authors examined whether Y. enterocolitica has the potential for acquiring antibiotic resistance from another organism in vitro. Mixed cultures of multiple-antibiotic-resistant Escherichia coli and a sensitive Y. enterocolitica yielded resistant Y. enterocolitica. Resistance to at least four drugs was transferred at a frequency of approximately 10(-7) transconjugants per donor. The newly resistant Y. enterocolitica transferred resistance to a sensitive E. coli at a slightly higher frequency. Resistance to at least one antibiotic was transferred to all five strains of Y. enterocolitica tested.