L-Asparagine is responsible for a major portion of the effect of peptone on the growth of L5178Y lymphoblasts in culture and is essential for the reproduction of these cells. Among the compounds tested, the requirement for the amide is replaced only by certain peptide derivatives of asparagine, and these are less active on a molar basis than is L-asparagine; L-aspartic acid, D-asparagine, and L...