The presence in nuclei and cytoplasm of cultured BHK-21/C13 cells (baby hamster fibroblasts) of RNA species with high affinity for poly(A) was detected using either a Millipore-poly(A)-binding assay or columns of poly(A)-Sepharose 4B. The nuclear species which labels rapidly is metabolically unstable and appears to be a specific subclass of heterogenous nuclear RNA. Digestion with T1 RNAase gives rise to a low level (1%) of oligonucleotide fragments of a disperse size range which are relatively rich in uridylate residues. The cytoplasmic species with affinity for poly(A) is similar in size to polyadenylated messenger RNAs and also associates with polysomes. However it appears to be distinct from the polyadenylated messenger RNAs by virtue of an unusual base composition and relative metabolic instability.