We tested the ability of antimicrobial agents that act on the cell wall to kill enterococci and found defective killing (a minimal bactericidal concentration/ minimal inhibitory concentration ratio of -32) with both ,8-lactams (penicillin G and cephalothin) and non-,B-lactams (vancomycin, cycloserine, and bacitracin We thawed these cultures and subcultured them first on blood agar plates and then on nutrient agar slants (Remel), which were used to inoculate overnight cultures for both the identification and susceptibility studies. These isolates were initially identified as enterococci by growth in 6.5% salt broth and by the hydrolysis of esculin in the presence of 40% bile (4, 11). Subsequent identification was performed by the methods of Facklam (4) and Gross et al. (5).We studied 10 viridans group streptococci and 9 Streptococcus bovis strains that had been isolated from blood cultures during the same time period and stored similarly at -70°C. We also tested four S. bovis isolates kindly provided by Robert C. Moellering, Jr. These strains had been initially identified by alpha hemolysis with negative tests for the hydrolysis of sodium hippurate (1) and esculin (viridans group streptococci) or by the hydrolysis of esculin in the presence of bile and failure to grow in 6.5% salt broth (S. bovis).The group D precipitin reaction was determined with group D antiserum (Difco) by the method of Rantz and Randall (12), with group D antigen (Difco) as a positive control. We used 40-ml overnight cultures in Todd-Hewitt broth (Difco) and lysozyme treatment (5 mg/ml for 4 h at 3700) (21) for bile-esculin-positive strains that failed to react with the group D antiserum after the Rantz and Randall procedure (one S. bovis isolate).We used commercially prepared bile-esculin medium, 6.5% salt broth, arginine decarboxylase medium, and litmus milk (Remel). Potassium tellurite (Difco), 2,3,5-triphenyltetrazolium (Sigma) agar, 2% starch agar, and heart infusion broths for acid production from carbohydrates were prepared by the method of Facklam (4). Carbohydrates tested were D-sorbitol, Dmannitol, D-melibiose, L-arabinose (Sigma), and Dglycerol (Difco). Broth for acid production from sodium pyruvate (Sigma) was prepared as described by Gross et al. (5). Todd-Hewitt broth was used to grow overnight cultures for the identification tests, MuellerHinton broth (Difco) was used to grow the inocula for susceptibility testing, and sheep blood agar plates (Remel) were used to subculture tubes without visible growth in the tube dilution studies of antimicrobial susceptibility.Antimicrobial agents. The antimicrobial agents used were penicillin G (Pfizer), cephalothin (Lilly), vancomycin (Lilly), cycloserine (Sigma), and bacitracin (Sigma