Probiotics are living microorganisms that, when administered in adequate amounts, confer health benefits on the host. Many studies indicate that there are several ways in which probiotics, which include lactic acid bacteria (LAB), have health benefits. These bacteria survive in a wide range of temperatures and pHs, from storage to digestion. In this study, I investigated the effect of varying pH and temperature on LAB growth in a model stomach with MRS broth, one of the most common growth mediums used by microbiologists to grow LAB. I isolated LAB from Yogourmet® yogurt starter, which contains L. Bulgarius, S. Thermophilus, and L. Acidophilus, and grew the bacteria in MRS broth at temperatures of 4°C, 22°C, and 37°C to imitate conditions under which various probiotic products are stored. I also grew LAB at pHs of 2, 4, 6.2, and 8 to replicate variations of pH that come with digestion. I measured bacterial growth at optical density (OD) 600 after 24 hours for all trials. I found that optimal LAB growth at the standard pH of the MRS broth (6.2 ± 0.2) and at the standard temperature of the human stomach (37°C) was expected and confirmed through several trials. When conditions deviated from standard, there was always less LAB growth. I concluded that variations in pH and temperature from standard stomach conditions result in smaller amounts of probiotic growth. This finding is important because a smaller amount of bacterial growth in the stomach will result in reduced health benefits on the host.