The cumulative effect of peptidase and protease activities associated with cells of Streptococcus thermophilus (ST) and Lactobacillus delbrueckii subsp. bulgaricus (LB) was evaluated on the milk protein-based antimicrobial peptides casocidin and isracidin. Reaction mixtures of casocidin or isracidin and nonproliferating mid-log cells of these essential yogurt starter cultures were individually incubated for up to 4 h at pH 4.5 and 7.0, and samples removed at various time points were analyzed by reverse phase-high performance liquid chromatography (RP-HPLC) and MALDI-TOF/TOF-MS. Both casocidin and isracidin remained largely unchanged following exposure to cell suspensions of ST or LB strains at pH 4.5. Casocidin was extensively degraded by both ST and LB strains at pH 7.0, whereas isracidin remained largely intact after incubation for 4 h with ST strains but was degraded by exposure to LB strains. The results showed the feasibility of using the bovine casein-based peptides casocidin and isracidin as food grade antimicrobial supplements to impart fermented dairy foods additional protection against bacterial contamination. The structural integrity and efficacy of these biodefensive peptides may be preserved by timing their addition near the end of the fermentation of yogurt-like dairy foods (at or below pH 4.5), when conditions for bacterial proteolytic activity become unfavorable.