In 1950, Morgan, Morton and Parker described one of the first totally defined media that did not depend largely on animal products or extracts as nutritive sources. This medium supported the growth of primary chick embryo heart cells. It has since become known as Medium 199. Its current use is with the addition of serum in virus and vaccine production, and in the culture of many non-transformed cells. It is also used in combination with less complex media. Medium 199 modified with Hanks’ Balanced Salts is designed for use with cells maintained in ambient (non-CO2) atmospheric conditions. M2852 differs from the original formulation by containing all L-amino acids. The original formulation contained 10 DL-amino acids, which are less biologically active on a per gram basis than the corresponding L-amino acids.