Eight strains of mutants streptococci and two strains of Actinomyces viscosus were studied to determine (1) their relative sensitivities to fluoride, lithium, and strontium and (2) whether lithium or strontium interact to enhance the known antimicrobial effects of fluoride. Analyses of variance of the data revealed a major inhibition of growth and acid production by fluoride, but not by lithium or strontium. Fluoride (5 mg/l) inhibited cell growth of all strains tested by a mean of 17% and total titratable acid production by a mean of 31%. However, there were marked differences between the strains. Mean total titratable acid was inhibited by fluoride least with the A. viscosus strains (15%) and most with the mutans streptococci (34%). Although interactions among the elements were statistically significant, they were generally slight in magnitude.