Summary The avian RNA tumour virus structural protein p12 was purified from avian myeloblastosis virus (AMV) by nucleic acid affinity chromatography to apparent homogeneity as judged from SDS-polyacrylamide gel electrophoresis. A filter-binding assay was used for the identification of p12. High concentrations of p12 precipitated nucleic acids out of solution in the absence of MgCl2. Binding of p12 to single-stranded nucleic acids protected them from digestion with nucleases and resulted in a hyperchromic effect. These phenomena were reversible in the presence of salt. The affinity of p12 to nucleic acids was determined by competing for the binding of p12 to denatured radioactive DNA by various other nucleic acids. It was found that p12 bound preferentially to single-stranded nucleic acids and showed a higher affinity to poly(rI) than to poly(rC) and poly(rA). Purified RNA-dependent DNA polymerase activity from AMV was stimulated up to sixfold by p12, depending on the template. Solubilization of RNA in RNA-DNA hybrids by RNase H was inhibited in the presence of p12.