MUCH interest has centred recently on the discoveries of ribohomopolymer sequences in RNAs of eukaryotic cells. Poly(A)1–3, poly(U)4 and poly(G)5 regions have been found in nuclear and cytoplasmic RNA fractions. These sequences are thought to have special implications for regulation of biological functions of RNAs. It seems likely that these ribohomopolymer sequences are synthesised in a post-transcriptional process. Poly(A) polymerases have been reported6–7,13 and enzymatic activities catalysing short (two to five nucleotides) chains of poly(U) have been reported in cytoplasmic and microsomal fractions in rat liver8,9. Enzymatic activities which polymerise four different ribohomopolymers independently have been found in nuclear ribonucleoprotein particles10.