Functional probiotic food products should contain live cells of probiotic strains. Their count is also important. In the FAO/WHO guidelines on the viability of probiotic bacteria in food products, including Lb. acidophilus strains, the therapeutic minimum has been defined (the count of live Lactobacillus bacteria) as 10⁶ cfu/cm³ of the product. Based on numerous investigations, it can be concluded that not all the probiotic strains meet the criterion of therapeutic minimum. The objective of this paper was to determine changes in the count of Lb. acidophilus in the nonfermented and fermented milk during four weeks of its refrigerated storage and to determine factors impacting the reduction of the bacteria count. The impact of fermentation process, environment of incubation, and oxygenation of the environment on the viability of Lb. acidophilus was proved during a period corresponding to the expiry period of fermented milk products.