Native small ribosomal subunits in mouse ascites tumor cells prepared by a sucrose zone sedimentation and analyzed by CsCl isopycnic centrifugation, consisted of four kinds of particles which could be distinguished reproducibly by their ultraviolet absorption. These particles had buoyant densities at 1.40, 1.42, 1.46 and 1.49 g/cm3 and were designated as S1, S2, S3 and S4 particles respectively. Studies on labeling kinetics with radioactive RNA precursors and the pulse label--actinomycin D chase experiment, demonstrated that radioactivity was incorporated first in precursor particles with a density of 1.45 g/cm3 and then appeared in S3, S1 and/or S2 and S4 particles. Results on labeling after administration of actinomycin D and EDTA treatment of the labeled small subunits revealed that S1 and S2 particles contained a messenger-like ribonucleoprotein particle and this particle plus tRNA, respectively. Since appearance of newly formed S4 particle was remarkably delayed and the amount of it was reduced after incubation of the cells in vitro with 10 mM sodium fluoride, this particle originated from dissociation of the small subunit.