A repeating structure of cytoplasmic poly(A)-ribonucleoprotein is revealed by digestion with T2 RNase. A pattern of fragments that are multiples of about 27 residues is obtained. The repeating structure is readily reconstituted from purified poly(A) and cytoplasmic factors. Reconstitution is specific for poly(A), as shown by the lack of competition by poly(G), poly(C), poly(dA), and tRNA. The repeating structure is absent from the nucleus, and so appears to be formed upon transport to the cytoplasm.