Previous studies have demonstrated a direct iron-irreversible inhibition of a variety of microorganisms by human apolactoferrin. The present study compared the bactericidal effects of lactoferrin on Streptococcus mutans with the bacteriostatic effects of iron deprivation. Growth (as determined by change in optical density) and macromolecular synthesis, as determined by incorporation of ¹⁴C-labeled uracil, thymidine, and lysine, were inhibited by incubation of washed exponential-phase S. mutans NCTC 10449 with purified human apolactoferrin. Similarly, apolactoferrin inhibited glucose uptake and metabolism. Iron-saturated lactoferrin had no effect on bacterial growth or metabolism and was capable of serving as a source of iron in iron-depleted medium. S. mutans failed to grow, and there was no indication of macromolecular synthesis in iron-depleted partially defined medium; however, glucose metabolism continued, though at a reduced rate, and viability was retained for 72 h. There was no detectable metabolism of glucose by cells maintained for 18 h in iron-free medium. Metabolism was restored by transfer of iron-depleted S. mutans to iron-complete medium. This was in contrast to the irreversible inhibition by lactoferrin after 1 h of incubation. Inhibition could not be reversed by removal of cell surface-associated lactoferrin as detected by rhodamine isothiocyanate-labeled antilactoferrin. This inhibition of metabolism and rapid loss in viability observed with lactoferrin treatment suggest that lactoferrin has a direct bactericidal effect on S. mutans that cannot be attributed to simple iron deprivation.