The complete amino acid sequence of the nucleic acid binding protein p10 of the Rickard strain of feline leukemia virus (FeLV) was determined. Fragments obtained by enzymatic digestion were purified by reverse-phase liquid chromatography and subjected to semiautomated Edman degradation. FeLV p10 is a basic polypeptide composed of 57 amino acids of MW 6604. The structure of p10 is compared to the structures of other retroviral nucleic acid binding proteins, and an analysis of a highly conserved region, the putative binding domain, is presented.