Disruption of the previously described efaA (from Enterococcus faecalis antigen A) gene was generated in E. faecalis strain OG1RF and loss of an 37-kDa immunoreactive band from the mutant was demonstrated in Western blots. In a mouse peritonitis model, mice infected with the efaAfs (fs=from Enterococcus faecalis) mutant showed more prolonged survival than mice infected with the parent strain OG1RF. These results suggest that efaAfs encodes a function important for infection of mice by enterococci. An efaA-like gene was also identified in E. faecium DNA libraries and its deduced amino acid sequence showed 73% similarity to EfaA of E. faecalis and 42-63% similarities to a group of streptococcal virulence and adhesion associated proteins that are components of ATP-binding cassette transport systems. Intragenic probes representing efaAfs, recAfs, efaAfm (fm=from E. faecium) and gyrAfm were tested for their ability to identify E. faecalis and E. faecium using colony lysates of 133 enterococci and one Streptococcus sp. Probes of E. faecium and E. faecalis origin hybridized to all isolates of E. faecium and E. faecalis, respectively, regardless of their clinical source but not to any of 29 other enterococci. These results suggest that the use of gene probes may prove helpful in identification of isolates of E. faecium and E. faecalis.