The antibiotic susceptibility of 10 strains of non-enterococcal group-D streptococci was compared with that of 20 strains of viridans streptococci. The minimal inhibitory concentrations of penicillin, ampicillin, oxacillin, nafcillin, cephalothin, vancomycin, erythromycin and clindamycin for the two groups of streptococci were very similar in range and median values. Both groups of streptococci were resistant to the aminoglycosides. The effect of the combination of penicillin, ampicillin or vancomycin with streptomycin, kanamycin, gentamicin or tobramycin on the in-vitro killing of the two groups of streptococci was compared. For all the antibiotic combinations tested, synergism was demonstrated against all strains of non-enterococcal group-D streptococci after one or more of the time-intervals 6, 24 and 48 h. Some or all of the antibiotic combinations were synergistic against all strains of viridans streptococci after one or more of the same time-intervals. The other aminoglycosides (kanamycin, gentamicin and tobramycin) offered no advantage over streptomycin in synergism with penicillin, ampicillin or vancomycin against nonenterococcal group-D streptococci or viridans streptococci. These results suggest that non-enterococcal group-D streptococcal endocarditis may be treated by the same regimen as endocarditis caused by the viridans streptococci.