RNA labeled with 32P was extracted from mouse ascites tumor cells, and centri-fuged in a linear sucrose gradient. After a pulse of 30 minutes most of the label was in a “heavy” (>30 s) and a “light” (4 to 10 s) fractions. The base composition of the fractions differed depending on whether or not sodium dodecyl-sulfate was included in the phenol extraction procedure. Without sodium dodecylsulfate the radioactivity of the “light” fraction predominated. This fraction contained a high proportion of cytosine which disappeared after treatment with phosphodiesterase. After the enzymic treatment, the base-ratios resembled those of transfer RNA. The “heavy” fraction differed from ribosomal RNA by a low proportion of adenine and a high proportion of uracil. More radioactivity was recovered after a short pulse when sodium dodecylsulfate and phenol were used. The “heavy” fraction differed from ribosomal RNA (28 8) only by slightly higher adenine and uracil contents. A DNA-like RNA was separated from transfer RNA contained in the light fraction (4 to 10 s). DNA-like RNA was also directly separated from DNA fibers. After labeling for 4·5 to 7 hr RNA was uniformly labeled, but there were still differences in the base ratios of the fractions. Ribosomal 26 to 28 s RNA had lower adenine and uracil contents than ribosomal 16 to 18 s RNA.