DL-GIyceraldehyde, a known inhibitor of glycolysis which does not appreciably af fect respiration, has been found to depress the incorporation of labeled leucine into pro tein of normal and neoplastic tissues, in vitro. With Yoshida ascites hepatoma cells, the inhibition of amino acid incorporation into protein occurred both in aerobic and anaerobic conditions, the L-isomer being apparently more effective than the Disomer, at comparable concentrations. In anaerobic conditions, DL-glyceraldehyde

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... and L-glyceraldehyde markedly inhibited glycolysis as well as amino acid incorporation into protein. In the same conditions, D-glyceraldehyde and a,/8-dihydroxybutyraldehyde (3-methylglyceraldehyde) inhibited incorporation more effectively than glycoly sis. The addition of glucose to the incubation medium partially counteracted the inhibitory effect of DL-,L-, and D-glyceraldehyde on the aerobic incorporation of amino acid into protein and relieved, at least in part, previously established inhibition. Glyceraldehyde failed to show appreciable effects on the concentrative capacity of the cell for a-aminoisobutyric acid, a model amino acid which shares a common transport system with several naturally occurring amino acids. These results are compatible with the hypothesis that glyceraldehyde inhibits glycolysis and amino acid incorporation into protein by independent mechanisms.