When serum is made rate‐limiting for clonal multiplication of human diploid fibroblasts, the presence of a 2‐oxocarboxylic acid in the medium becomes essential. The requirement is independent of the 20 amino acids and glucose. Glyoxylic, pyruvic, 2‐oxoglutaric, and oxalacetic acids are most effective. The types of 2‐oxocarboxylic acids that support multiplication are oxidized substrates for several, pyridine nucleotide‐linked intracellular oxidoreductases. The requirement is not satisfied by carboxylic acids, oxidized substrates for oxidoreductases that are not linked to pyridine nucleotides, or by nonspecific electron acceptors. The quantitative requirement for 2‐oxocarboxylic acids in cell multiplication is markedly affected by the concentration of serum proteins in the medium. Therefore, 2‐oxocarboxylic acid metabolism may be related to the mechanism by which serum growth factors regulate cell multiplication. Copyright © 1979 Wiley‐Liss, Inc.
