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Yoshida ascites hepatoma cells growing in vivo are characterized by a large number of monosomes and dimers which further increase when the cells are incubated anaerobically in vitro in the absence of glucose: under the latter condition most of the ribosomes are present as inactive particles. These monosomes can be incorporated into polyribosomes if the elongation step of protein synthesis is inhibited by cycloheximide. The degradation of polysomes in the absence of glucose under anaerobic conditions can be reversed promptly on addition of glucose to the medium and less promptly by oxygenation. These data suggest that the large monosome-dimer pool of hepatoma cells depends on a relative inefficiency of the initiation step of protein synthesis, which is magnified by the lack of energy caused by anaerobic incubation in the absence of glucose, and counteracted by interference with further steps of protein synthesis or conditions which favor glycolysis or, less efficiently, respiration of the tumor cells.