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A low molecular weight, highly basic DNA-binding protein was purified from several oncornaviruses by the sequential procedures of gel filtration in guanidine-hydrochloride, DEAE-cellulose chromatography and affinity chromatography on single-stranded DNA sepharose. The binding protein from Rauscher and woolly monkey type-C viruses was the fastest migrating of the virion proteins in SDS-polyacrylamide gels and thus is designated p10 according to previous convention although our estimates of molecular weight were 8-9,000 daltons. The binding protein from these two viruses was resolved into two bands by acid-urea electrophoresis although only a single NH2 terminal amino acid was detected (S. Oroszlan, personal communication), thus indicating charge heterogeneity. Antibody to Rauscher virus p10 species-specific in gel diffusion and complement-fixation tests and did not exhibit cross-reactivity with other virion proteins. A DNA-binding protein was also detected in preparations of mouse mammary tumor virus. This purified protein had an apparent molecular weight of 12,500, was the second fastest migrating component in the virus preparations, and was antigenically unrelated to the mouse type-C virus p10.