The in vitro bactericidal interactions of penicillin G, cefotaxime, or vancomycin in combination with gentamicin were compared against 20 group G streptococci by the timed kill curve method. Synergy was noted at the following frequencies: penicillin plus gentamicin, 80%; cefotaxime plus gentamicin, 85%; vancomycin plus gentamicin, 90%. There was no bactericidal antagonism observed.