M199 Medium (low or medium glucose, low sodium bicarbonate, L-glutamine, phenol red, without HEPES) is widely used in cell culture applications, particularly for the growth and maintenance of primary cells, including fibroblasts, endothelial cells, and epithelial cells. Its balanced composition supports the nutritional requirements of a variety of cell types, making it suitable for research in virology, immunology, and cancer studies. The low or medium glucose and low bicarbonate levels in this formulation make it ideal for environments with controlled CO2 levels, ensuring stable pH conditions for cell growth. Additionally, the inclusion of L-glutamine provides a critical energy source for cells, while phenol red acts as a pH indicator, allowing researchers to monitor the health and conditions of the culture. This filter-sterilized medium is often employed in studies requiring precise control over cellular environments, such as drug screening, tissue engineering, and vaccine development.