A patient with enterococcal endocarditis, who relapsed after repeated courses of apparently adequate treatment with ampicillin plus gentamicin, was subsequently cured with ampicillin-tobramycin therapy. The organisms isolated from this patient were strains of Streptococcus faecalis that were resistant to penicillin (or ampicillin)-gentamicin synergism but not to penicillin (or ampicillin)-tobramycin synergism. The mechanism of resistance in these strains appears to be related to a specific defect in the intracellular uptake of gentamicin (but not tobramycin) in the presence of penicillin.