199 medium was designed by Morgan et al. in 1950, initially for nutritional studies of chicken embryo fibroblasts. It is now widely used for culturing various animal cells, including some non-mammalian species. 199 is particularly suitable for culturing non-transformed cells and is frequently employed in virology, vaccine production, and the cultivation of rat pancreatic epithelial cells and mouse lens tissue. Compared to other basal media, 199 contains unique components including adenine, adenosine, hypoxanthine, thymidine, and other vitamins. It features two balanced salt formulations: Earle's salt composition is commonly used in CO₂-supplemented environments, while Hank's salt composition is employed in non-CO₂ conditions.