To the Editor: Dr. Koenig, in his letter published in the June 3 issue of the Journal, is correct in stating that there is laboratory evidence that a combination of penicillin and streptomycin is more effective than either agent alone in killing enterococci in vitro. The factors involved in the enhanced bactericidal effect of this combination are, to the best of my knowledge, unknown, and the isolated environment of the culture tube is not necessarily even an approximation of the conditions existing within an endocarditic vegetation.Support for the validity of the in vitro observations has come from clinical experience, . . .