The Walker carcinosarcoma 256 has been shown to require twelve amino acids in addition to glutamine and asparagine (11) for growth in tissue culture. Also, glycine and serine were markedly stimulatory to these cells. The “nonessential” amino acids—alanine, proline, hydroxyproline, aspartic acid, and glutamic acid—were neither essential nor stimulatory. All amino acids tested were inhibitory to some extent at 10–20 mm. A basal medium containing the fourteen essential amino acids or amides supplemented with serine and/or glycine supported excellent growth.