Mechanisms for the possible transfer of antimicrobial resistance genes between staphylococci and enterococci remain poorly defined. We have previously reported the transfer between Enterococcus faecalis strains of a multiresistance chromosomal element (beta-lactamase positive and resistance to erythromycin, gentamicin, mercuric chloride, streptomycin, and tetracycline) which we have tentatively designated Tn5385. Tn5385 is a composite of several smaller transposable elements, including Tn5384, a 26-kb composite transposon conferring resistance to erythromycin, gentamicin, and mercuric chloride. Analyses of 7 kb within Tn5384 and flanking sequences within the larger element revealed sequences characteristic of staphylococcal beta-lactamase and small, mobilizable plasmids flanking a region with a sequence identical to those of the replication genes previously described for enterococcal and streptococcal broad-host-range plasmids. These diverse regions are linked by insertion sequences IS256 and IS257 in a manner which suggests a series of cointegration events as the genesis of the current relationship. Taken together, these data suggest that Tn5384 and the larger element within which it is incorporated (Tn5385) evolved at least in part as a result of cointegration between an enterococcal broad-host-range plasmid and staphylococcal beta-lactamase and small mobilizable plasmids. These results implicate broad-host-range plasmids in the transfer of resistance determinants from staphylococci to enterococci.