In the present work, the effect of milk supplementation on viability of yogurt bacteria (Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus) and probiotic bacteria (Lactobacillus acidophilus and Bifidobacteria) was studied during yogurt manufacture and thirty three days storage. Incubation time to reach pH value of 4.5 was greatly affected by the addition of casein fraction of milk proteins. The viable counts of L. delbrueckii subsp. bulgaricus and Bifidobacteria were increased in yogurt supplemented with tryptone and milk powder plus five fold starter culture. Addition of 500 mg L-1 of cysteine, promoted the growth of L. acidophilus until three weeks from the date of production. Bifidobacteria counts remained more than 105 cfu ml-1 in yogurt supplemented with 2% milk powder and inoculated with five fold starter culture. Using a high level of inoculums promoted the viability of L. acidophilus and Bifidobacteria significantly (p< 0.05) in the first week of storage. پرونده مقاله