The size of poly (A)-RNA from polysomes and cell nuclei of the yeast Saccharomyces cerevisiae was investigated. Pulslabelled cells ([14C] adenin) were cracked by the French press; polysomes and nuclei were separately isolated and the RNA was finally extracted with phenol. The separation of poly (A)-containing and poly (A)-lacking fractions was achieved by oligo (dT) cellulose. These fractions were characterized by sedimentation analysis. The main portion of polysomal poly (A)-RNA sedimented with a rate of 8 to 14S, whereas the poly (A)-RNA of nuclei exhibited a sedimentation rate of 12 to 17S. Thus nuclear poly (A)-RNA is about 20-30% larger than polysomal poly (A)-RNA.