Combination of carbenicillin and cephalothin with various aminoglycosides (gentamicin, sisomicin, tobramycin, kanamycin and amikacin) were studied in vitro using 20 strains of enterococci isolated from clinical material. The addition of the aminoglycosides to carbenicillin increased the rate of bactericidal action of carbenicillin on most strains. Little benefit was obtained by adding the aminoglycosides to cephalothin and antagonistic action has been observed on some strains.