Native subribosomal cytoplasmic particles, labelled in vivo with [14C]orotic acid or with [32P]phosphate, have been isolated in quantity from rat liver and hepatoma by centrifugation in a zonal rotor. The particles were concentrated by membrane filtration and examined by resedimentation through sucrose gradients. The RNA contained in the particles was extracted and examined by electrophoresis and base‐ratio analysis. In liver, the 40 S region contained two distinct ribonucleoprotein particles, which are almost certainly two forms of the small ribosomal subunit. In addition, particles containing rapidly labelled, polydisperese, DNA‐like RNA were concentrated in this region. The 60 S region also contained some RNA not attributable to ribosomal subunits and was deficient in the latter. Hepatoma differed from liver in that its 40 S region was homogeneous on resedimentation and contained much less polydisperese RNA. The tumour also showed a much higher proportion of large to small ribosomal subunits.