ABSTRACT: The growth rate of two probiotic strains with documented health effects was studied in ultra-high temperature treated milk separately and beside each other. The probiotic strains were Lactobacillus acidophilus (La-5) and Bifidobacterium lactis (Bb-12). Fermentation was followed for 12 h at 37 ˚C and the samples were analyzed for pH, titratable acidity and viable counts of probiotic bacteria before incubation and at 2h intervals throughout the fermentation period. The obtained results showed that Lb. acidophilus had higher activity and growth rate than B. lactis when they were purely cultured in milk. Investigation on mixed culture inoculating the milk revealed that Lb. acidophilus had a stimulating effect on the growth rate of B. lactis, however, its growth wasn’t significantly affected by the presence of the latter bacteria.