The reovirus lambda 1 protein, a major component of the inner capsid, has been shown to exhibit an affinity for dsRNA in a 'Northwestern' filter-binding assay. In the present study it was demonstrated that the protein can bind dsDNA as well as dsRNA. A bacterial expression system was used to study the protein region able to bind to nucleic acids. The amino-terminal 187 amino acids of lambda 1 were fused to the bacterial maltose-binding protein and shown to be sufficient for binding to nucleic acids. The putative zinc finger present on lambda 1 is not encompassed in this fragment of the protein. Site-directed mutagenesis also indicated that this zinc finger motif is unrelated to binding. In contrast, mutations introduced in a previously suggested nucleotide-binding motif almost completely prevented the binding. These data indicate that the amino-terminal end of lambda 1, encompassing its nucleotide-binding motif, is involved in the affinity of this protein for nucleic acids.