Minimum inhibitory concentrations have been determined for ampicillin and seven cephalosporins against 93 strains of group D streptococci isolated recently from clinical material. Ampicillin was much the most active compound (modal MIC = 1.6 mug/ml); cephaloridine, cephacetrile, and cefazolin had a modal MIC of 25 mug/ml, while corresponding figures for cephalothin, cephradine, cephalexin, and cefoxitin were 50, 100, 200, and 800 mug/ml, respectively. Thus, none of the newer cephalosporins is an improvement in respect to activity against enterococci over existing compounds, and ampicillin remains overwhelmingly the beta-lactam antibiotic of choice for the treatment of infections by such organisms. Pharmacokinetic considerations, however, indicate that certain cephalosporins, for instance, cephaloridine, cefazolin, and cephanone, may be worthy of further study in view of possible synergy with aminoglycoside antibiotics.