Viability of yogurt starter cultures and Bifidobacterium animalis was assessed during 28 d storage in reduced-fat yogurts containing 1.5% milk fat supplemented with 1.5% fructooligosaccharide or whey protein concentrate. These properties were examined in comparison with control yogurts containing 1.5% and 3% milk fat and no supplement. Although fructooligosaccharide improved the viability of Streptococcus thermophilus, Lactobacillus delbrueckii subs. bulgaricus, and Bifidobacterium animalis, the highest growth was obtained when milk was supplemented with whey protein concentrate in reduced-fat yogurt (P < 0.05). Supplementation with 1.5% whey protein concentrate in reduced-fat yogurt increased the viable counts of S. thermophilus, L. delbrueckii subs. bulgaricus, and B. animalis by 1 log cycle in the 1st week of storage when compared to control sample. Similar improvement in the growth of both yogurt bacteria and B. animalis was also obtained in the full-fat yogurt containing 3% milk fat and no supplement. Addition of whey protein concentrate also resulted in the highest content of lactic and acetic acids (P < 0.05). A gradual increase was obtained in organic acid contents during the storage.