A single-strand-specific, nucleolar exoribonuclease from Ehrlich ascites tumor cells has been isolated and purified free from other nucleases. The exonuclease degraded single-stranded RNA processively from either a 5'-hydroxyl or a 5'-phosphorylated end and released 5'-mononucleotides. The enzyme digested single-strand poly(C), poly(U), and poly(A) equally well but did not degrade duplex poly(C).poly(I) or poly(A).poly(U). Less than 0.2% of duplex DNA or 1.5% of heat-denatured DNA was degraded under the conditions which resulted in greater than 26% degradation of RNA. The ribonuclease required Mg2+ (0.2 mM) for optimum activity and was inhibited by ethylenediaminetetraacetic acid but not by human placental RNase inhibitor. The native enzyme had a Stokes radius of 42 A and a sedimentation coefficient (S20,w) of 4.3 S. From these values, an apparent molecular weight of 76 000 was derived by using the Svedberg equation. The localization and unique mode of degradation suggest a role for the 5'----3' exoribonuclease in ribosomal RNA processing.