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Chinese hamster cells (line CHO) were cultured in F-10 medium from which isoleucine was omitted and added sera were dialyzed. This medium, which is deficient in isoleucine content in comparison to normal F-10 medium but not totally lacking isoleucine, is designated Ile-medium. The effect of reduced isoleucine concentration on DNA synthesis was measured in terms of thymidine incorporation and in terms of DNA content. Culture in isoleucine-deficient medium for 1 hr caused a decrease in thymidine incorporation to 44% of control (Ile+); by 30 hr thymidine incorporation in Ile-cultures was 1 of control or less. Cell division ceased before 30 hr in Ile-medium. The DNA content at 30-hr Ilewas found to be that of a GI cell and remained unchanged during further culture in Ile-. The effect on RNA synthesis was less pronounced. At 30-hr Ile-, uridine incorporation into RNA was 55-70% of control. RNA content was equal to or slightly greater than that of a mid-G1 cell. RNA content increased during continued culture in Ile-(15% from 30-to 48-hr Ile-). RNA species whose synthesis continued included 4 S, Hn, messenger, and ribosomal at rates approximately onethird, three-fourths, one-half, and one-fifth of exponential culture values, respectively. Distribution of pulse-labeled (uridine) species among cytoplasmic materials of differing densities closely followed that found in Ile+ cells. The size distribution of messenger synthesized in Ile-cultures followed that of Ile+ populations. Superformation of polysomes by culture in low levels of cycloheximide showed excess messen-

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