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In previous work it has been shown that end‐product regulation of ornithine‐δ‐transaminase in liver and kidney is mediated by different amino acids. It is now described that the enzyme purified from liver has different properties from that obtained from kidney with regards to temperature optimum, rate of heat inactivation and Kmvalues. In HeLa cells grown in suspension and synchronized with thymidine, the “liver” form of ornithine‐δ‐transaminase appears in the S phase, while the “kidney” form of ornithine‐δ‐transaminase appears in the G1 phase. In the G2 phase there seems to be some mixing of the two enzyme forms. These data and reciprocal oscillation of proline content per cell and ornithine‐δ‐transaminase activity in the M and G1 phases suggest that only in these stages of the cell cycle the enzyme is subjected to the same type of regulation as in kidney.