Data derived from testing the bactericidal activity of combinations of penicillin with gentamicin or streptomycin and of clindamycin with gentamicin on nine isolates of Streptococcus mutans were analyzed by preparing isobolograms to determine the presence of additive, synergistic, or antagonistic effects. Synergy with penicillin-aminoglycoside combinations was found in two strains; additive effects occurred in seven instances with penicillin-gentamicin combinations; and antagonism occurred in eight instances with clindamycin-gentamicin combinations.