The function of reverse transcriptase (RT) in retroviruses is to copy their RNA genomes in DNA to be integrated into the host genome. We report a high purity preparation of this enzyme by adsorption onto poly(U)-sepharose. We analyzed RT activity with poly(rA)--oligo (dT)12-18, poly(dA)--oligo (dT)12-18, poly(rC)--oligo (dG)12-18 and poly(rCm)--oligo (dG)12-18 to obtain a more accurate pattern of RT. 8 out of 16 malignant breast samples were positive for RT activity, more specifically with poly(rC)--oligo (dG)12-18 and poly(rCm)--oligo (dG)12-18. The 5 non-malignant mammary samples tested were negative for RT activity. Assays with poly(rC)--oligo (dG)12-18 and poly(rCm)--oligo (dG)12-18. The 5 non- malignant mammary samples tested were negative for RT activity. Assays with N--nitroso--N--methylurea--induced rat mammary carcinomas were negative for RT activity.